[Bobfelter]

If the computer was previously overheating & you left it on unattended, then it is quite possible that you smoked wither the CPU or the motherboard. The symptoms you describe are consistent with a CPU damaged from overheating.I recommend removing the CPU and inspecting it for signs of overheating. What you look for depends on what processor you have. In general, look for black spots on the bottom of the cpu or bubbled areas on top of the CPU.Then reinstall the CPU. Make sure the heatsink & fan are mounted securely on top of the processor. Then try to boot it again.I have seen many overheated AMD chips that caused the motherboard & CPU to both fry.