Chances are it's not the PSU. In my experience, these die completely, by which I mean that it would be very unusual indeed for it to give even a split second's worth of power.
Firstly, unplug everything and take it outside. Then remove as much of the case as you can and give it a serious clear out with a canister of compressed air.
Then, gently rock any expansion cards in their slots to check that they're securely seated. Do the same with the RAM chips.
If you're then certain that the PSU is faulty, start here:
http://www.adventsupp...wtopic.php?f=8&t=9160