I am trying to rebuild a Gateway Astro All-in-one PC. It won't power up: no bios, beeps or anything; but when I take the "bottom half" of the system off - everything but the monitor and PSU - plug it back in the "top half" powers up. It's been in storage for ~six months; it worked great before then. So I'm going to replace the mobo battery in the morning. What else could be wrong?