Battlefield bad company 2 Cd version, old serial key, new computer

Hi! I have this problem. I bought the game 2011 and installed it on my old pc, sold the pc a while ago and unistalled everything from the old one. I am now trying to install the game again but the online serial key is ''already in use''. How do I get a new one? Do I have to buy one or can I get a new one from EA somehow by proving that I have the game?