Well my first thought is that it was NEVER the switch. Presuming that you mean the on/off button on the front of the case.
That is only a contact switch, it simply shorts out two pins on the motherboard header
The power switch on the front of the case is just a momentary contact switch. The motherboard always has a little bit of power, and pushing the contact switch simply completes a circuit between two little prongs on the motherboard that signals the power supply to start supplying full juice to the system. When you open the case and trace the wires from the backside of the on/off switch down to the motherboard, you can pull the those two wires off the motherboard and touch the two prongs with a small screw driver (be careful poking around - touch only the two prongs that were underneath the wires from the switch). The system should come to life. If so, then the switch is bad, it's not making the momentary contact to complete the circuit as it should..
If you still have the old power button, connect a multimeter on a resistance reading, or a continuity if the meter has that - the switch does not have to be connected to anything and which way round you connect the meter leads does not matter. Work the switch. If you are on resistance if the meter is digital 1 is open circuit and when you hold it in 000 or close to it depending on the scale is closed circuit.
Further to the foregoing, it may and I stress MAY be either a PSU transient defect or far worse a motherboard issue.
Please DO NOT think that I am saying it is, but I strongly suspect it is NOT the switch.
If the power to the computer is connected via an anti-surge bar it is worthwhile trying a straight connection to a power outlet.
I have known the anti-surge bars with a transient defect
Edited by Macboatmaster, 23 September 2011 - 11:24 AM.