If you bought a valid Home Use Program key (I have had similar in the past), try to attempt activation via the phone robot instead of directly online. Did you download and install the copy you downloaded?
If it still fails, call MS and talk to someone by voice.
Do you still have the original email with the key?
Go through the windows activation if it fails it will show contact numbers to call Microsoft. If you have had this key on another PC you may have to buy another copy but I would contact Microsoft to find out.
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.