After I had done some changes in Windows 10 to make it faster and less "chatty" to Microsoft (switch off Telemetry, switch off error reporting etc.) I could not use search any more. Symptoms were:

• Can't type into Cortana search field
• Can't type to search in Start Menu
• Can't type to search in Win 10 App Store
• Can't type in taskbar search
• Can't type in Settings search

There are lots people with the same or similar issues as can be seen from a simple Internet search "win 10 taskbar search not working".

Unfortunately, from all the suggestions, nothing worked for me:

• Re-install Cortana (keyboard search should also work without Cortana, so that did not help)
• Kill the Cortana/Search process
• Rebuild the search index
• Re-install all default apps
• Restart the "Windows Search" service
• Run the "Search and Indexing" Troubleshooter
• Run system file checker (SFC) to search for currupted files
• Re-install the keyboard driver

After some hours of poking around, I found that I could right-click in the search field and paste the contents of the clipboard! And I saw the correct search results. So the index must have been fine. It looked like a keyboard issue, not a search issue.

But the keyboard worked in any application (e.g. Notepad, Excel etc.) just not in the searches. I started to suspect that it could be a Windows 10 "Modern Apps" issue. As I never use Modern Apps, I first had to load one (I chose "Calculator"), and - Bingo! - I could not enter any numbers via the keyboard. I could only click on the numbers, but could not type them.

I found that the keyboard did not work for any Windows 10 Modern Apps, just for the traditional desktop apps. Finally I found through some Internet searches how to solve this problem:

1. Open Task Scheduler
2. Go to Microsoft > Windows > TextServicesFramework
3. On MsCtfMonitor, do a right-click and enable the task
4. Restart your machine

That's it. All the search boxes worked again!

Update: If the task MsCtfMonitor is missing from the list of tasks, you can import it with the .XML file below:

MsCtfMonitor.xml

Thanks to Rob for the tip.