Now this is a difficult problem. I have a customer with Windows 8 who could not activate Windows. The primary problem was a HP computer with the product key embedded in the UEFI. (Which means, of course, no product key sticker. One of my many reasons to hate Windows 8.) Everything I tried to activate Windows failed. I even used a Magic Jellybean keyfinder to recover the product key.
Before you start the repair process, be sure you have the Windows 8 product key.
Techs-on-Call Computer Repair Notes 