Take the motherboard out and set it on top of the box it came in. Place the antistatic bag ontop of the box and then the mobo ontop of the bag.
Now hook up power, RAM, vid card, and keyboard only.
Try to boot up...if it does, your motherboard is grounding out.
Remove just the vid card and boot up. If nothing happens, it's not the video card, if the system all of a sudden beeps (of course you need to have the mobo close enough to your case so you can keep the PC speaker attached to the mobo so you can hear these beeps) anywho..if it beeps..then the video card needs to be reseated or perhaps it's faulty.
If that does nothing, put the card back in and now remove the RAM and try booting again.
Same thing applies, if you get nothing then the RAM isn't causing the system hang up. Like with the card though, if your system beeps now, then either the RAM is dead or perhaps wasn't seated right.
Now if you've gone through those components and it still doesn't boot then it seems it could be the CPU or motherboard unfortunately. Either a motherboard will be completely dead or it will just power on but nothing happen. Unfortunately you basically gotta RMA both the CPU and motherboard unless you have another motherboard to try the CPU in and another CPU to try in the motherboard in question.