It could be a power supply problem. Power supplies have been my achillles heel of late...

Depending on the make and model of the machine, you may be able to just unscrew it or you may need to get a new case. Or it could be even worse. (I had an old Dell that had a non-standard power plug on the motherboard, so I had to replace the case and scour the internet for a guy who was building adapter cables in his basement)