The game might have changed the resolution. Try System Preferences > Display > Display tab > scroll to the bottom of the resolution list and select the bottom most entry.
(This assumes you're on 10.6.8 as your profile says. If you're on Lion (10.7.x), pick the top most entry.)
If the screen moves with the cursor and the menubar and dock are off the screen, it could be zoomed. In that case hold CMD and scroll down with the mouse or trackpad to unzoom.