In such cases I've seen MSFT to refund the purchase or issue a new key.
Keep in mind that there is no third party help on activation issues. You must deal with MSFT.
Back when they were first pushing Windows 8 in the stores I found a Pro version for $80 and had my parents buy it since I knew we were all going to upgrade components and OS in the near future. I just got done getting the upgrades done on the PC installed the new OS- no apparent issues. Went to update wont let it update. Checked and found despite entering the key while it was being installed, it didn't register. Tried activating it via the settings menu and it tells me the key is bad. Went online to look for help, everything is a download and basically a crack. We have a legit copy of W8 so I wouldn't have to do these other things. Can anyone offer me help, tried using the site and it was to say the least beyond frustrating. To boot that, it has me no closer to getting the issue solved. And yes, this is their seperate CD/key of Win8 Pro from what I installed on my desktop.