A common cause for a computer shutting down is overheating. First, unplug your computer and make sure there is no power coming into the computer. Remove the back screws, and take off the cover. Second, look at the fans. If they are dusty, use compressed air to remove the dust from the fans. Finally, put the cover back on, plug the computer power cord in, and press the power button on. This will solve many cases of overheating, but if you still have a problem, call Taleam Systems for Computer repair in Ottawa Ontario.