Sounds like you have a short. Please remove all of your components from the case, place them on a flat, non-conductive surface, reseat the RAM, ensure CPU is properly cooled, reseat all power connectors, then try to turn it on again.
The reason to take it out of the case is to eliminate the possibility of anything shorting out against the case, or something that could be wedged between them and the case. (like a screw that you may have dropped behind the motherboard)