Assuming the PSU is ATX (the question seems meaningless for AT).
I copied the following which might help from some Q&A webpage, can't remember the URL.
<i> Why the computer/fan does not turn on after I flipped the power switch to ON located on the ATX power supply?
ATX power supply is different from AT power supply. It depends a logic circuit on the motherboard to turn it on. Once you flipped the power switch on the ATX power supply to on (some ATX power supplies do not even have such a switch), the ATX power supply sends a 5V 720MA current to the motherboard through pin 9 on the power connector. That current is for WOL (Wake-up On Lan) and power on circuits. There is a power-on jumper on the motherboard that connects to the pushbutton located in front of ATX case. When the pushbutton is pressed, it sends a signal to the mothboard, which in turn notifies the ATX power supply to turn on the full power. The location of the power-on jumper on the motherboard is manufacture dependent. You will need to read your motherboard manual to locate that jumper. At any event, do not try to manually jump-start the power supply without attaching motherboard. Since the power supply is expecting certain sensing circuit feedback to regulate the output voltage, munaully starting it without attaching it to the motherboard could cause damage to the power supply. </i>
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