Yep, I've had this a few times... Have you added any new hardware? It could be a couple of things, in order: motherboard, psu, CPU. Since you have tried the psu, next thing would be to remove all but one stick of ram, remove video card and everything else that doesn't need to be there, then try start it up. If it works, then add the next component and try again.
Maybe re-seating you ram and power connections etc will help! If not, it's most likely your motherboard... Power surge or something may have caused some damage.
I have had this problem twice, once it was the motherboard, and once it was the psu.