Al wrote:

The MB circuit only tells the enable circuit inside the PSU to let the
PSU turn on. To what voltage does the PWR_ON (green wire) fall when you
push the ON/OFF switch? The MB must pull this wire lower than 1.5v for
the PSU to turn on. I have seen enable logic failure modes where, for
example, 1.2v would not turn the PSU on, but 1.0v would. Check that
voltage level before replacing any ICs on the MB.

When I hit the pwr button that voltage level doesnt change.....(and
the switch is fine)

Hmmm...could still be a short in the pull-up in the PSU, but I doubt it.
Likely something went bad on the MB, but why? It's not a usual failure.
How old is the MB?