assassin's creed2 code already been used ? :(

hi,
i'm a bit bored these days , so i installed some old games on my computer again... one of the is assain's creed 2 ... now i install the game , i have to logg on on a program called uplay (i still rember my passw and acc name so tht's no probelm). but when i wanna launch the game , they ask for the CD key , so i filled it in and what was the answer ? "code already been used" !!!!! wtf ... so now i can't play this game anymore or wut ? :o please help! :(
(AC2 white edition)

i hope some1 has a solution
thx