I have had the same problem once, I really haven't got a clue as to what made it happen. But I do know I solved the problem by reinstalling the game, without uninstalling it first. So what you do is when you start up the game tell it to install, and it will write fresh copies of some of the files onto your harddisk. With me that solved the problem.
If that doesn't work....Backup your saved games folder, and your (dl)update folder and uninstall the game. Use the registry cleaner that is on this site and install fresh. Put your saved games and (dl)update back in place and try again.