There are many discussions you can read about this. Just for starters try http://www.google.com/search?hl=en&q=DEAD+PC
My favorite is still cardboard and just the basic 4 parts of motherboard, CPU, heatsink+fan, speaker and the PSU. I hope it beeps or else I've knocked it down to those 4 parts.
I suspected it might be the powersupply at first.
I have an antec Phantom 350 powersupply. The thing is when I plug the power in the led on the front display for the clock and tempature reading goes on.
I see a green light for the network card turn on which means power must be going through my board. So I don't know if it's my powersupply. When I turn it on nothing happens.
BTW this occured because there was a poweroutage in my house for a moment when I was gone. I thought the circuit breaker was tripped due to a surge but there was nothing tripped.