Why won't it start

I have a new power supply and a new hard drive. The computer will not turn on. The green power light is on and the orange light on the mother board is on solid. When I turn on the computer (front button) the green power light in the back of the computer goes out and nothing happens. I tested the power supply before I put it in also.Why can't I get this computer to fire up??? Its been awhile but It was solved....Its a bad motherboard........Thanks for the help guys....