Prolly because the game is still set to a resolution the new monitor can't display.
And don't think you have to restart your computer, just hit ctrl+alt+del.
Erm, have to go by memory, but I think there's a folder for the game in your "Documents" folder. In there there's a main.cfg, which contains a few settings such as resolution. Try setting the proper resolution there manually.
Maybe delete the file; a lot of games generate it automatically when it's not present. Make a backup though, in case it throws an error.