Press Windows key
Enter cmd , you will see " Command Prompt", run it as administator
Enter the following command:
wmic path SoftwareLicensingService get OA3xOriginalProductKey
(you can paste it with RMB or ctrl+v)
Press Enter
The product key will be revealed, copy the product key.
Click Start > Settings > Update & security > Activation > Change the product key then enter your product key and click Next.
Probably after this you will get an error but you will see a new button: activate by phone
Try activating windows by phone.