Windows 10 wiped my restore images now what?

I have to reinstall windows from a disc because the windows 10 upgrade totally <ADMIN NOTE: Profanity removed as per TOU> up my desktop. Meanwhile i cant reinstall windows because i have no reg key on the computer to input it. what do i do now? If i cant get into certain areas of the pc because of this upgrade issue how do i get my windows reg key for windows 8? I need help