Last year I upgraded my Windows 8.1 license to Windows 10 via the free upgrade programme.
Yesterday I needed to reinstall Windows 10. I booted into the USB to install and I entered my Windows 8.1 product key, and it says my key is not accepted.
This is on exactly the same hardware with no changes whatsoever.
EDIT Downloading and using the latest Windows 10 ISO and signing in using my Live account solved the problem.