I take it that you did try a different
PSU.
The standby power (the orange light!) is actually produced by a different part of the
PSU, so if the main part of the
PSU has gone, then you may well still get the standby power.
Other than that, it may be that the voltage supply for the
CPU on the mobo has been toasted, or part of the chipset died... ):
Áedán