This article explains why this error happens, what not to do, and the legitimate (and legally complicated) methods to get your game running.
Fix for "CD Key already registered" at EA Nation - Steam Community serial number nfs shift 2 unleashed fix
Provide your proof of purchase (Steam receipt or Origin history) to have them manually refresh your key. After Install, Game Doesn't Start [Fix] - Steam Community This article explains why this error happens, what
Copy your serial number from Steam and use the "Redeem Code" feature in the Clean Reinstall: what not to do
If you can still access the game's files, you should de-authorize your current machine before reinstalling or moving to a new PC to "save" an activation slot.
