disconnect everything, take the motherboard out and put in on a cardboard box so it won't ground out.
Plug in only the keyboard, 1 stick of RAM and the video card (and of course CPU
Make sure the case is close enough so you can plug in the PC speaker for beeps.
Try to turn it on, if it comes on, it was being grounded out in the case
If it doesn't, take out the graphics card see if it gives you a beep code, if it does, then reseat the card and try it again, if it doesn't work after reseating then your card is bunk.
Repeat the same process with the RAM....if that doesn't work then it's either the CPU or mobo and the only way you can find out which it is, is if you had a spare mobo to put the CPU in