If it "twitches" but does not start, this means your power supply detected an overload (or underload) and shut back down. This can be a bad power supply or a defective device attached to it or the motherboard. You troubleshoot by detaching things, pulling boards, etc., to see if the PS and CPU fans will run. The PS does need a load of some type so you cannot run it by itself. You can easily troubleshoot down to it's being the PS or motherboard but not beyond that without special tools.