PC won't turn on. It seems it isn't a power-supply problem..

Greetings everyone...

The reason I'm writing this is because I'm having some problems with
one of my computers. It all happened when I shutdown the machine, it
shutdown normally, but the next time I tried to turn it on, it didn't.
It seemed like a power supply kind of problem, since, it did not do
anything at all. Trying to turn it on, was like trying to turn on a
computer whose cable is unplugged. I switched its power supply with
another one I had laying around. But, still... it did exactly the
same thing. I'm sure the power supply works, because I tested it on
other computer. Could it be that the motherboard is damaged? How can
I know the problem? If you any of you guys could point me out to a
online hardware troubleshooting guide (or perhaps enlight me with
your knowledge and experience), it would be most appritiated. Sorry
if this thread seems a bit dumb, but I rarely work with PC hardware
or anything. I try to take best care out of my machines and its rare
that they have (cost-effective, fixable) hardware problems.

Again, thanks for the future replies and help any of you may bring.
Cheers.