how to find windows product key after losing installation media

I built my own PC with windows 7 OEM. I was able to upgrade to windows 8, but I've since lost the installation media. I want to install windows 10, but I don't have the product key. I found a script online that purports to give you the product key from the registry, but when I tried the script on my laptop, and used the given key during installation on my laptop (which didn't come with a key for some reason), it said it was invalid.

Am I doomed to forever suffer with windows 8.1?