Happened to me yesterday. Control+Alt+Delete into task manager and kill the executable was the only way out that I found. If you are on a Mac or a console I have no idea, maybe the reset button.
cheers for the replies lads, I ended the process and restarted ok.....unfortunately for some reason I then tried to test my Asps ability to fly through a sun....it doesn't do too well, one emergency stop later and I'm cussing my stupidity!