sounds like a dead short. make sure you used the brass standoffs for the case so that the mb is not shorted to the case. also check that the i\o shield for the mb is not grounding out one of the ports or the mb. also check on the back of the mb that if your using a 3 party heat sink if they used a metal plate the plate not tuching the case or the mb and shorting the mb out.
also make sure you connected the 8 pin power plug on the top edge of the mb. most new power supplys uses a 4 by 4 plugs to make 8 pins. if you uses a 6 by 2 you uses the pci video power plug and it wont work. when you power the mb up check to see if the cpu or ram fault led is on hard. if it just the ram led try using one stick and make sure the ram stick is seated.
a lot of times the ram stick or the cpu may be crushed with to much force or the cpu not seated right. if it not a dead short check that the cpu is seated and there no bent pins. (may want to bread board the mb outside the case. ).