You say the PSU works fine? How do you know that? Are there any leds lit on the motherboard? network card? Does it run but just won't display a picture? If so then it's probably the video card or a bad ram module. Did you plug it in on a different circuit in the house? Try bridging the power switch pins on the motherboard with an insulated handle screwdriver, but be very careful.

If you have absolutely no power, then your problem is the power supply. They are rather inexpensive and pretty easy to install.

Gary