Sometimes upgrading to Windows 10 will make our license server think that you have attempted to register your license to a new machine. 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 this happens, login to your account and go to the Your Licences page. You can use the Manage Licence Usage option to remove the old registration for your PC. Your license should then be accepted.