1.- Press the Home key and type "CMD".
2.- Right click on “Command Prompt” and select “Run as administrator”. (Note: You can use Powershell as an alternative if you prefer.)
3.- Enter the command:
slmgr –ipk <Your Windows product key>
4.- Then, enter the following command:
slmgr -ato
5.- Restart the PC and if it has not been activated, click on the problem solver and it will be activated.
Good luck.