Couple of points:
Modern computers are NEVER actually fully off.
The power supply provides some parasitic power for things like the power on switch, the network interface to permit network controlled power on etc.

As part of this whole system, the PS supplies a signal to the mobo called "Power Good" - if this signal is not present, the power on switch will not do anything.

So, an easy check might be to plug the PC into an Ethernet switch - if the power supply is functioning correctly then the Ethernet port should show a link light, even when the PC is "OFF". If there is none in the port on the PC, then check the Ethernet switch, there will surely be one there. If the link light is present but the PC won't switch on, then it is probably not the PS, but either the switch/wiring OR something on the mobo itself.


--=-- My credo: If it's worth doing, it's worth overdoing - just ask my missus, she'll tell ya laugh --=--
You're only paranoid if you're wrong!