Im guessing you get no POST on screen just a quick flash from the tower, I had the same issue i while back when building my PC...
All i done in the end was make sure everything is seated in place properly, Even if it meant stripping it right back.. I took all connectors off pins/plugs, Unplugged PSU and removed the RAM, Put it all together again and i found out i connected a couple of the Front panel connections onto the wrong pins..
I know its a pain but you could always try that.