I have a 7 year old Gateway desktop that was working perfect until the other day. It suddenly stopped but the fan was still going. I unplugged the computer and I went inside the case and disconnected the power supply and hard drive then reconnected them. I turned the computer on, then a beep, and it was fine. When I shut down the computer did not start up again. So I disconnected the power supply and hard rive and then it powered up again. What's going on here? How do I fix it, if at all?