um, are you talking power switch or PSU? If the switch, then just short the power pins to complete the circuit and see if it powers on. If it powers on its either the physical power button mechanism (if this is the switch you are talking about), or wires. You may also want to disconnect all uneeded periphs/cards. If this is right, someone go for me. At work and hard to get back at times. Course, this first thing you should do is check that it is plugged directly into a wall outlet (always go for the easiest thing first). Another thing to check is what the power switch is set to on the back of the PSU (110/220 etc).
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