Help! ATX PC doesn't turn on

Last week I was working with my ASUS A7V133 when it suddenly came to a
strange state: the front power light was blinking in the same way as if
it were sleeping. But nothing would make it alive, only this 4 sec press
on the power button would turn it off... and never on.

Now, when I turn the rear switch off for enough time (enough for the
mainboard led indicating power to turn off) and back on, if I press the
power button, the front light will be on for 1/10 sec and then off.
Nothing inside will move.

This is pretty annoying for it doesn't seem to be a power supply failure
(the mainboard seems to be powered), but who knows with these ATX boxes?

Can anybody give me a hint?

Thanks in advance,

David.