I presume it is doing NOTHING on the button?
Try unplugging the main kettle adapter at the back of the power supply and check the plug fuse. Plug in again firmly and try the power button again.
If nothing and fuse ok, you may have a Power supply gone down (they usually go pop!) There 'is' a possibility (a small one) that is is a motherboard problem, but I would suspect the PS Unit. These are just a plug-in box with a few connections.
Take the side of case off and check the power supply rating(W) I would expect it to be a pretty standard ATX supply. Check the connectors YOU have connected .. look for one with same set. They have SATA or IDE type connections .. some power supplies have both.
Check this forum, which has pics ..
http://h30434.www3.hp...wer-Supply/m-p/250428
Plenty power supplies here ...
http://shop.ebay.co.u...d=p3286.c0.m270.l1313
Come back in case of difficulty.