I am assuming that you replaced you PSU 'thinking' that was the problem, but your MOBO <or> CPU can be the problem <or> if you'r lucky the power switch. To test the power switch - find the leads and short them with a paper clip/screw driver head {PW+ and PW-}.
If it is a NEW MOBO then it too may be shorted; to verify remove ALL mounting screws and dangle the MOBO and pull it away from the I/O shield and 'try' to power it on. You can also breadboard the MOBO by removing it and placing on cardboard.