Leave it unplugged for 2 - 3 days and see what happens. Sounds like a comparable issue with my 840AV, which worked perfectly if not used for a while but would not boot if you restarted or shut down/restarted. I switched the power supply to no avail, RAM, etc - and eventually replaced the caps, which resulted in a reliable machine that would fire up every time. The boards also seem to get really dusty in these cases, and the silver lining corrodes quite badly, so give the internals a good clean out too with compressed air.

JB