I reset computer, it was originally running Windows 8.1.

So, I reset my computer, it wasn't running too great and I just wanted a fresh start, now it says "Product ID: Unavailable" in the System Information section, and I can't upgrade to Windows 8.1 because I didn't see it in the store. I don't have a Windows 8 Product Key because the computer came already activated with Windows 8. I really need this to start working and I'm screwed if it...