Electrical shorts inside a computer are usually caused by stray pieces of metal that form an electrical connection that shouldn't normally exist. Electrical shorts can cause the PC to power off without warning and without an error message. They can also cause the PC to not power on at all.
Warning: Always power off and unplug the PC before troubleshooting causes of electrical shorts. The computer should always be unplugged when working inside the case.
Electrical shorts inside the computer are often caused by stray screws in the case that have come in contact with the motherboard or other internal component. Screws are used to secure nearly every component to the inside of the case including video cards, sound cards, hard drives, optical drives, etc.
Pick up the computer case and gently rock it side to side. If you hear a rattling sound, a screw may have come loose and is rolling around inside your case. A few light shakes will usually knock it loose and onto the bottom of the case.
If the screw is lodged somewhere that you can't reach with your fingers, use a long pair of tweezers to remove it.