Pull the case cover off. Look at the 20 pin connector and find pin 14 (the green wire). One trick is the use of a paper clip: insert the reconfigured paper clip into the green wire (pin 14) and jumper to either pin 15 or 13 (the black ground wires). Make sure that when you insert the paper clip that you are 1) not holding it 2) the power supply is disconnected from the wall (or the rear of the PSU). Once the paper clip is inserted, make sure the power supply on/off switch is in the off positoin and plug in the power cable to the PSU or back into the wall. Then turn the power switch on at the PSU. If it is the Mobo power switch that is the problem, the system will power on. Essentially, what you have done is bypass the power switch on your case by jumpering the Mobo to "power on" at the 20 pin connector. If it is a 24 pin connector, read your mobo manual to determine which connector is the correct one to jumper.
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