Dead computer?

Computer in question had a problem when it last ran... Computer froze and
the only way they could shut it down was to unplug it. When they tried to
power it up again, nothing would happen. They took it to the person who
built it who proclaims, it's fried. She got it back from him (minus the
power supply because he says that's what fried). I said I would look at
it. So I put in a power supply. The cpu fan runs but nothing else on
power up (power light on case lights up). In order to shut it down, you
have to either unplug the power supply or turn the power supply off (it has
a switch). My question is.... did this thing fry MY power supply now (the
power supply fan runs)? Or, is there another motherboard problem that is
causing nothing else to power up (CD-Rom, hard drive, floppy) nothing
initializes. Could that be because the motherboard is dead and the BIOS is
not going through post? No beeps, nothing when you try to power up. My
knowledge of the system is limited, it is a homebuilt with a Pentium II
CPU. Motherboard has 3 ISA slots, 4 PCI Slots and 1 AGP Slot. No markings
on the motherboard, it has an Intel controller. I know I'm not offering
much info here, but any thoughts are greatly appreciated. Much thanks.

Patty