Pull the power lead out the back of the computer so it goes...

Pull the power lead out the back of the computer so it goes totally dead and wait till all the lights go out inside on the motherbaord, so wait 30 seconds ~ then plug it back in and try again..

if that doesnt work I'd just start unplugging stuff.. remove your DVD, try and turn it on... remove your hard drive try and turn it back on, if you have a sound card pull that, see if you can turn it on.. same with video card.. if a device has a short then you will know as it will turn on when you find it..

failing that try shorting the jumper on the motherboard your front case power button plugs into (its only a 12v circuit)

failing that maybe pull out the cmos battery for 30 seconds (with power to case removed) then plug back in..

if that doesnt work you will need to try another PSU .. or the mobo/cpu died..