Try pressing the computers "On" button, often they are setup to shutdown with a press of it. If not then you can hold in the button for approx 5 secs and the computer should switch off. Also if you have a reset button you can press that, failing all of these then switch off at the mains, When you restart it should work OK, if not then you will need to delve into restart problems for your OS. BTW what OS is it? W2K or XP?
Hood