Windows 10 Product Key Tool

I've recently had my laptop formatted at a computer shop after getting hard drive boot partition issues...
after formatting, the guy working there activated windows 10 with a fake code.. knowing that before formatting i had already upgraded to windows 10 and had a working windows 10 version, is it possible to reactivate windows 10 with the product key from your tool or do i have to reinstall windows 8.1 (the version my laptop shipped with), and upgrade to windows 10 ?, or i should just enter the product key in windows 10 ?
Thanks in advance..