If Windows is not automatically activated you can correct this in just a few steps, usually without the need of a reboot.
In command prompt:
Type: net stop slsvc (It may ask you if you are sure, select yes)
Type: rename tokens.dat tokens.bar
Type: cd %windir%\system32
Type net start slsvc
Type: cscript slmgr.vbs -rilc (It may take a long time for this to complete, please be patient)
The licence should now be able to be reactivated without a full reinstall.