Hello.
I first bought my game online, at 'D2D' (direct2drive.com). When I bought a new system I had to reinstall the game. My activation code didnt work, so i wrote to the site and they sent me a new activation key. Unfortunately this only works once or maybe twice, then your screwed.
Write to the site where you purchased it and describe the problem.
This actually happened to me too many times, I had to purchase it again because they wouldn't send me another activation code
.
Also, try writing to ubisoft, but they might tell you to contact the site you purchased it from. But maybe they will help.
-Search this site for posts, I remember others had the same problem