Stupid question, have you tried unplugging the power cord from the power supply, letting it sit for a min and trying to power it back on?

I'd wager power supply over anything. Can you see a brand name on the power supply?

EDIT: Actually yes. I'm going to say its definitely the power supply.

Reason: When the power supply turns on, it sends out a pulse on the grey-line of the main ATX power connector to the motherboard. This will tell the motherboard if the power is clean and good, thus OK'ing the system to fully power up. If the signal is not good it will..not turn on.

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