Windows 8 Reinstall / Product Key Trouble / Upgrade?

I have a VAIO core i5 laptop that died, seemingly of hard disk trouble. After trying lots of fixes (including using my bootable USB, which just wouldn't work), I finally managed to get the computer to initiate recovery via 'VAIO Assist' and to do some hard disk repair, and then it seemed to work like new. However, the next time I powered up the computer, I discovered that recovery had wiped Windows 8.1 Professional clear off the machine, leaving only Windows 8 Professional -- without a valid product key -- and now the machine won't let me do anything until I enter a 25-character product key. The computer was a gift, and I wasn't given a product key or back-up disks for either operating system, Windows 8 or Windows 8.1. I assume the only thing I can do is to buy a new key, and I'm totally fine with that assuming there's no other legitimate recourse. My question at this point is what should I buy -- Windows 8 Professional, Windows 8.1 Professional, Windows 10 Professional? And where? I don't want any more trouble or to waste any more time. I just want this computer to work without hassle. Any advice?