Occasionally upgrading to Windows 10 or installing a major Windows 10 update can make it appear like you are using a new PC.
If you have already used all of your allowed registrations, your license key will then be rejected, even though it is the same PC.
If that happens, login to your account and go to the Your Licenses – Purchases and Trials page. You can use the Manage Licence Usage option to Remove the old registration for your PC. Your license should then be accepted.