On motherboards that have the metal arm that contacts the battery (+) sometimes the heat of normal use can cause a small gap (size of human hair) just large enough to keep it from working. (especially if it was snagged by a cable or something, or during battery replacement) To see the gap you usually have to pull the MB out and hold it up to the light. If you see this gap and rebend the arm to correct it, you may still need to send the BIOS chip out for reflashing.
The fact that before it quit working it kept rebooting sounds like it was either running too hot, was overclocked, or has a wrong jumper setting. A lot of things can cause rebooting to happen. I would pull out all cards, reinsert video card only, and then try. If it boots up install one card at a time.