Are you getting a green light on the back of the PSU?
You could also try taking out all RAM, video card, additional add-in cards, basically everything except the power to the mainboard and power switch connector. See if anything happens then. If it does, work backwards until you isolate the component pulling the system down.
If you still have no luck, then I'd say the mainboard is dead. Generally buying a replacement mainboard for a propriety system such is this is cost prohibitive. I would suggest looking on ebay (or similar) for a 2nd hand replacement unit. There should be hundreds of them around at the moment, and prices should be good as supply generally is greater than demand.