HP A1616N desktop-no response

[mkcrks]

Have an HP that doesn't respond to power button. No fans or lites. Had PSU checked at Compusa and they say it's OK. There is a green led on the PSU that lights when I plug it in to 120V but when I plug the large connector into the mobo the led goes out. Have unplugged all unnecessary items to no avail. Any ideas would be appreciated.
Thanks
Mike

[GX1_Man]

So you are plugged into the wall WHILE you are plugging in the motherboard? 

Does the light still come on before this happens now?

What happened prior to this that was unusual

[mkcrks]

With power unplugged from mobo I plug in PSU to wall. Green led comes on. I unplug from wall, plug connector into mobo then plug into wall and the green led does not come on. It's almost like something on the mobo is shunting down the PSU. I've unplugged all components with the same results. Thanks for your response and any further info you can provide.
Mike