Maybe try unplugging it from the wall, waiting a few minutes, then plugging it back in. Make sure the switch on the PSU is set to the correct voltage in your area, and that the power switch it set to on. If that still doesn't work, take a big whiff of the PSU and see if it smells burnt at all.
Do you have surge protection on that system? I'm just suggesting things that target the PSU first, then branching off from there. Also, did you try switching around the wiring from the mobo to the power and reset switches on the case? You can grab a screwdriver and bridge the contacts of the power switch pins on the board to turn it on, see if that works.
Edit - if you have some time, I've solved many a problem by tearing apart the system, reducing it to a pile of parts on my desk, and rebuilding it from the ground up....just a thought.
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