diff --git a/_locales/en/messages.json b/_locales/en/messages.json
index 8cc5b47..72eadeb 100644
--- a/_locales/en/messages.json
+++ b/_locales/en/messages.json
@@ -28,8 +28,8 @@
         "description": "about"
     },
     "back": {
-      "message": "Back",
-      "description": "Back"
+        "message": "Back",
+        "description": "Back"
     },
     "version": {
         "message": "Version",
@@ -354,5 +354,9 @@
     "hidden_content_note": {
         "message": "NOTE: When <code>Hidden content?</code> is checked, the extension will search through all elements regardless of their visibility, which may result in a higher match count.",
         "description": "Note about hidden content behavior"
+    },
+    "pageUnreachable": {
+        "message": "Search and Replace cannot reach this page. Reload the page and try again.",
+        "description": "pageUnreachable"
     }
 }
diff --git a/src/constants.ts b/src/constants.ts
index b840716..01912d7 100644
--- a/src/constants.ts
+++ b/src/constants.ts
@@ -13,6 +13,16 @@ export const RICH_TEXT_EDITORS = {
  */
 export const ELEMENT_FILTER = /^(HTML|HEAD|SCRIPT|STYLE|IFRAME)$/i
 export const INPUT_TEXTAREA_CONTENT_EDITABLE_SELECTOR = 'input,textarea,*[contenteditable="true"]'
+
+/** The bundle listed under content_scripts in manifest.json, and injected on demand */
+export const CONTENT_SCRIPT_FILE = 'searchreplace.js'
+
+/**
+ * Chrome's wording when `tabs.sendMessage` finds nobody listening in the target tab, which
+ * happens when the page was loaded before the extension, or when the extension has been
+ * reloaded or auto-updated since the page was loaded, orphaning its content script.
+ */
+export const NO_RECEIVER_MESSAGE = 'Receiving end does not exist'
 export const HINTS: Record<string, Hint> = {
     gmail: {
         hint: 'Hint: Gmail detected. Check "Input fields only?" when editing draft emails.',
diff --git a/src/cypress/fixtures/example.json b/src/cypress/fixtures/example.json
new file mode 100644
index 0000000..519902d
--- /dev/null
+++ b/src/cypress/fixtures/example.json
@@ -0,0 +1,5 @@
+{
+    "name": "Using fixtures to represent data",
+    "email": "hello@cypress.io",
+    "body": "Fixtures are a great way to mock data for responses to routes"
+}
diff --git a/src/popup.ts b/src/popup.ts
index 5615160..32ab3af 100644
--- a/src/popup.ts
+++ b/src/popup.ts
@@ -28,6 +28,7 @@ import {
     manifest,
     tabConnect,
 } from './util'
+import { deliverToContentScript } from './popup/deliver'
 import { getRemainingMatchCount } from './popup/count'
 
 // Enum for content types
@@ -382,6 +383,11 @@ async function formSubmitHandler(
     await storeTerms(true)
     // do the search replace
     await contentScriptCall(action, searchReplaceInstance, historyItems)
+    if (!reachedContentScript) {
+        // No reply is coming, so take the spinner down and say why
+        removeLoader()
+        setPageUnreachableHint(translationFn)
+    }
 }
 
 /** Send the search and replace instance to the content script for processing, clearing any saved responses first.
@@ -408,12 +414,27 @@ export async function contentScriptCall(
             url: tab.url,
             instanceId,
         }
-        await chrome.tabs.sendMessage(tab.id, message)
+        // Injects the content script and retries if the page does not have one. Never rejects
+        // for a missing content script, so a page the extension cannot reach leaves the popup
+        // usable instead of hanging on a spinner.
+        const delivery = await deliverToContentScript(tab.id, message)
+        if (delivery === 'no-content-script') {
+            reachedContentScript = false
+            return undefined
+        }
+        reachedContentScript = true
         url = tab.url
     }
     return url
 }
 
+/**
+ * Whether the last contentScriptCall reached a content script. The response arrives
+ * asynchronously through chrome.runtime.onMessage, so the popup has to remember that the
+ * request never got anywhere in order to stop waiting for a reply.
+ */
+let reachedContentScript = true
+
 /** The callback function for the content script
  * @param msg {SearchReplaceResponse}
  * @param translationFn
@@ -467,6 +488,24 @@ function setHints(hints?: Hint[]) {
     }
 }
 
+/**
+ * Tell the user the page cannot be reached, in the same place hints appear. Reached when a page
+ * has no content script and one cannot be injected — a chrome:// page, the Web Store, a PDF, or
+ * a file:// URL without file access.
+ */
+function setPageUnreachableHint(translationFn: TranslationProxy) {
+    const hintsElement = document.getElementById('hints')
+    if (!hintsElement) {
+        return
+    }
+    hintsElement.innerHTML = ''
+    const alert = document.createElement('div')
+    alert.className = 'hint alert alert-warning'
+    alert.setAttribute('role', 'alert')
+    alert.innerText = translationFn('pageUnreachable')
+    hintsElement.appendChild(alert)
+}
+
 function removeLoader() {
     const loader = document.getElementById('loader')
     if (loader) loader.style.display = 'none'
diff --git a/src/popup/deliver.test.ts b/src/popup/deliver.test.ts
new file mode 100644
index 0000000..78a3c9d
--- /dev/null
+++ b/src/popup/deliver.test.ts
@@ -0,0 +1,110 @@
+import { ChromeMock, installChromeMock } from '../testing/chromeMock'
+import { SearchReplaceContentMessage } from '../types'
+import { beforeEach, describe, expect, test } from 'vitest'
+import { deliverToContentScript, isMissingContentScript } from './deliver'
+
+const TAB_ID = 7
+
+function message(): SearchReplaceContentMessage {
+    return {
+        action: 'searchReplace',
+        instance: {
+            searchTerm: 'find',
+            replaceTerm: 'replace',
+            options: {
+                matchCase: false,
+                inputFieldsOnly: false,
+                hiddenContent: false,
+                wholeWord: false,
+                isRegex: false,
+                replaceHTML: false,
+                replaceAll: true,
+                save: false,
+            },
+        },
+    }
+}
+
+let mock: ChromeMock
+
+beforeEach(() => {
+    mock = installChromeMock()
+})
+
+describe('isMissingContentScript', () => {
+    test('recognises the error Chrome raises when nothing is listening', () => {
+        // The wording the reporter of issue #126 saw in the extension's error log
+        expect(isMissingContentScript(new Error('Could not establish connection. Receiving end does not exist.'))).toBe(
+            true
+        )
+    })
+
+    test('does not swallow unrelated errors', () => {
+        expect(isMissingContentScript(new Error('The tab was closed'))).toBe(false)
+        expect(isMissingContentScript(new Error('Extension context invalidated'))).toBe(false)
+    })
+
+    test('copes with something thrown that is not an Error', () => {
+        expect(isMissingContentScript('Receiving end does not exist')).toBe(true)
+        expect(isMissingContentScript(undefined)).toBe(false)
+    })
+})
+
+describe('deliverToContentScript', () => {
+    test('sends straight through when the content script is present', async () => {
+        expect(await deliverToContentScript(TAB_ID, message())).toBe('delivered')
+        expect(mock.sentTabMessages).toHaveLength(1)
+        // No need to inject when the page already has it
+        expect(mock.executedScripts).toEqual([])
+    })
+
+    test('injects the content script and retries when nothing is listening', async () => {
+        mock.tabsWithoutContentScript.add(TAB_ID)
+
+        expect(await deliverToContentScript(TAB_ID, message())).toBe('delivered-after-injecting')
+        expect(mock.executedScripts).toEqual([{ tabId: TAB_ID, files: ['searchreplace.js'] }])
+        // The first attempt failed, the second succeeded
+        expect(mock.sentTabMessages).toHaveLength(1)
+    })
+
+    test('reports no content script when the page cannot be injected into', async () => {
+        // chrome://, the Web Store, a PDF viewer, or file:// without file access
+        mock.tabsWithoutContentScript.add(TAB_ID)
+        mock.blockedTabIds.add(TAB_ID)
+
+        expect(await deliverToContentScript(TAB_ID, message())).toBe('no-content-script')
+        expect(mock.sentTabMessages).toEqual([])
+    })
+
+    test('does not reject when the page cannot be reached, so the popup stays usable', async () => {
+        mock.tabsWithoutContentScript.add(TAB_ID)
+        mock.blockedTabIds.add(TAB_ID)
+
+        await expect(deliverToContentScript(TAB_ID, message())).resolves.toBe('no-content-script')
+    })
+
+    test('reports no content script when injection succeeds but the retry still fails', async () => {
+        // The tab navigated away between injecting and sending
+        mock.tabsWithoutContentScript.add(TAB_ID)
+        mock.injectionDoesNotHelp.add(TAB_ID)
+
+        expect(await deliverToContentScript(TAB_ID, message())).toBe('no-content-script')
+        expect(mock.executedScripts).toHaveLength(1)
+    })
+
+    test('passes other errors on rather than hiding them behind an injection', async () => {
+        mock.tabMessageError = new Error('Extension context invalidated')
+
+        await expect(deliverToContentScript(TAB_ID, message())).rejects.toThrow('Extension context invalidated')
+        expect(mock.executedScripts).toEqual([])
+    })
+
+    test('delivers the message unchanged', async () => {
+        mock.tabsWithoutContentScript.add(TAB_ID)
+        const msg = message()
+
+        await deliverToContentScript(TAB_ID, msg)
+
+        expect(mock.sentTabMessages[0]).toEqual({ tabId: TAB_ID, message: msg })
+    })
+})
diff --git a/src/popup/deliver.ts b/src/popup/deliver.ts
new file mode 100644
index 0000000..6176f4e
--- /dev/null
+++ b/src/popup/deliver.ts
@@ -0,0 +1,55 @@
+import { CONTENT_SCRIPT_FILE, NO_RECEIVER_MESSAGE } from '../constants'
+import { SearchReplaceContentMessage } from '../types'
+
+/** Whether a rejection from `tabs.sendMessage` means no content script was listening */
+export function isMissingContentScript(error: unknown): boolean {
+    const message = error instanceof Error ? error.message : String(error)
+    return message.includes(NO_RECEIVER_MESSAGE)
+}
+
+export type DeliveryResult = 'delivered' | 'delivered-after-injecting' | 'no-content-script'
+
+/**
+ * Send a message to the content script, injecting it first if it is not there.
+ *
+ * Chrome only injects content scripts into pages loaded *after* the extension is installed,
+ * updated or reloaded. Because Chrome updates extensions silently in the background, a
+ * long-lived tab — a dashboard someone leaves open for days — loses its content script without
+ * anything visibly happening. Every later `tabs.sendMessage` then rejects with "Receiving end
+ * does not exist", the popup does nothing at all, and the rejection surfaces as an uncaught
+ * error. That is the failure reported in issue #126 on Meta Ads Manager, and it explains a
+ * report of "it worked in the past, and a year ago it stopped".
+ *
+ * Rather than asking the user to reload, inject the content script and try again.
+ */
+export async function deliverToContentScript(
+    tabId: number,
+    message: SearchReplaceContentMessage
+): Promise<DeliveryResult> {
+    try {
+        await chrome.tabs.sendMessage(tabId, message)
+        return 'delivered'
+    } catch (error) {
+        if (!isMissingContentScript(error)) {
+            throw error
+        }
+    }
+
+    try {
+        await chrome.scripting.executeScript({
+            target: { tabId, allFrames: true },
+            files: [CONTENT_SCRIPT_FILE],
+        })
+    } catch {
+        // A page the extension has no access to: chrome://, the Web Store, a PDF viewer, or a
+        // file:// URL without file access granted
+        return 'no-content-script'
+    }
+
+    try {
+        await chrome.tabs.sendMessage(tabId, message)
+        return 'delivered-after-injecting'
+    } catch {
+        return 'no-content-script'
+    }
+}
diff --git a/src/testing/chromeMock.ts b/src/testing/chromeMock.ts
index 37c20d9..d1986f9 100644
--- a/src/testing/chromeMock.ts
+++ b/src/testing/chromeMock.ts
@@ -94,6 +94,12 @@ export interface ChromeMock {
     executedScripts: { tabId: number; files: string[] }[]
     /** Tab ids for which `chrome.scripting.executeScript` should reject, as restricted pages do */
     blockedTabIds: Set<number>
+    /** Tab ids with no content script listening, as a page loaded before the extension has */
+    tabsWithoutContentScript: Set<number>
+    /** Tab ids where injecting does not help, e.g. the tab navigated away mid-flight */
+    injectionDoesNotHelp: Set<number>
+    /** Set to make `chrome.tabs.sendMessage` reject with something other than a missing receiver */
+    tabMessageError?: Error
     chrome: typeof chrome
 }
 
@@ -107,6 +113,9 @@ export function createChromeMock(options: { version?: string } = {}): ChromeMock
     const tabs: { id?: number; url?: string }[] = [{ id: 1, url: 'https://example.com/' }]
     const executedScripts: { tabId: number; files: string[] }[] = []
     const blockedTabIds = new Set<number>()
+    const tabsWithoutContentScript = new Set<number>()
+    const injectionDoesNotHelp = new Set<number>()
+    const state: { tabMessageError?: Error } = {}
 
     function respondTo(message: unknown): unknown {
         const action = (message as { action?: string } | undefined)?.action
@@ -145,6 +154,13 @@ export function createChromeMock(options: { version?: string } = {}): ChromeMock
         tabs: {
             query: () => Promise.resolve(tabs),
             sendMessage: (tabId: number, message: unknown) => {
+                if (state.tabMessageError) {
+                    return Promise.reject(state.tabMessageError)
+                }
+                if (tabsWithoutContentScript.has(tabId)) {
+                    // Mirrors Chrome's wording when no content script is listening
+                    return Promise.reject(new Error('Could not establish connection. Receiving end does not exist.'))
+                }
                 sentTabMessages.push({ tabId, message })
                 return Promise.resolve()
             },
@@ -159,6 +175,10 @@ export function createChromeMock(options: { version?: string } = {}): ChromeMock
                     return Promise.reject(new Error('Cannot access contents of the page'))
                 }
                 executedScripts.push({ tabId: injection.target.tabId, files: injection.files })
+                if (!injectionDoesNotHelp.has(injection.target.tabId)) {
+                    // Injecting gives the tab a listener, so a retry now succeeds
+                    tabsWithoutContentScript.delete(injection.target.tabId)
+                }
                 return Promise.resolve([])
             },
         },
@@ -180,6 +200,14 @@ export function createChromeMock(options: { version?: string } = {}): ChromeMock
         tabs,
         executedScripts,
         blockedTabIds,
+        tabsWithoutContentScript,
+        injectionDoesNotHelp,
+        get tabMessageError() {
+            return state.tabMessageError
+        },
+        set tabMessageError(error: Error | undefined) {
+            state.tabMessageError = error
+        },
         // The mock deliberately implements only the surface this codebase uses
         chrome: mock as unknown as typeof chrome,
     }