HP a402n

I have an HP a402n in which my power switch light on the front was flashing and the green light on the power supply was also flashing and the computer would NOT do anything. I took out the power supply (250) and replaced it with a working one (450). Upon turning the computer on, the power light on the front became a steady light and would not shut off, but the computer still would not come on. After trying different things, the light will now not come on at all, nor the computer still will do nothing....... with a working/tested power supply. Any suggestions on what MAY have happened? or should I just begin the search for a new mobo? Any help would be greatly appreciated!
Thanks a bunch!!