Where did you buy the original copy?
Whatever you do, do not be tempted to follow the advice in the preceding post (which is almost certainly Spam) and buy a key from some unknown Internet site. This is a classic scam and you will be throwing good money after bad.
The advice to talk to Microsoft is the correct path to follow. If the software is genuine they will fix your problem; if not they will want to know where you purchased it, and may well grant you an amnesty and activate it anyway.