Why is Windows 10 suddenly asking me to Activate (Error 0xC004C003)

I upgraded last year from Windows 8.1 which was licensed.

Since then I've been using Windows 10 quite happily with no problems although Windows Update reports it hasn't updated since May 2016 and that it can't update now due to problems with error 0x80004002 reported.

Last night I installed Photoshop CC 2015 which recommended I reboot before installing so I did.

I rebooted and installed Photoshop, checked that everything worked, then shut down.

This morning I boot up and now all of a sudden Windows isn't Activated.

Settings says the following:

The product key used on this PC didn't work with this edition of Windows. You might need to get in touch with the store or company that you bought Windows from, enter a different product key, or go to the Store to buy genuine Windows. Error code: 0xC004X003

I'm very confused as to why this has worked for more than year now but suddenly doesn't.