That is weird. Do you have an old disk, or the Steam version of the game?
Assuming that you have the Steam version, go to the Steam main window and click on the "library" tab. Then find the game and then right-click on the game name and click on "properties". Then click on "local files". And then on "verify integrity of game files...".
Then try again.
If that doesn't help, try to start a new game and see if you'll have the same problem again.
The last thing to try is to uninstall the game. Then reinstall the game and then try again. With this option, you might want to find your saved game file(s) and move them somewhere else. Then move them back later.