My PC became unactivated.

EdTittel said:

Apparently the key you provided and the edition of Windows you're running don't match. If you can't provide a key that's valid for the version you're running, you'll have to reinstall the version you have keys for. What keys do you have? Looks like you've got Pro installed, so I'm guessing you have Home licenses? Please elaborate on your situation so that extremely knowledgeable folks like @Superfly can chime in.
HTH,
--Ed--

My windows 10 is upgraded from Windows 8 Pro within the first year Windows 10 released, so it is a valid free license for Windows 10 Pro, and have no problem with activation for 2 years. However, I recently recovered my Windows 10 from a system image with Acronis True Image. I am not sure if it is the cause as I use Acronis True Image many years without problems.

Try opening a Command Prompt (Admin) - or "Run as Administrator" Command Prompt and run the following two commands and post the results here:

slmgr /ato
slmgr /dlv

Or book a support phone call from Microsoft. I replaced my CPU, motherboard and RAM on Saturday (first upgrade since 2010 ) and my Windows deactivated. I spoke to a very nice lady in India who remoted onto my PC, took a screenshot of the receipt for all the new bits and finally provided me with a Win 10 Pro activation key. I had to re-install because I was on an insider build but a few minutes later I was up and running and legit again :).