try completely takeing it apart only leave the PSU and the mother board in the case, take everything else out.make sure the heat sink is seated properly and the ram isn't loose. if it starts up then add in peice by piece and see what happens. vid card first, hard drive, sound card.... if even at bare bones it won't do anything try switching out the power supply or re checking your wirein. you could also have a short from not mounting the motherboard properly to the case.