Help - pc not turning on

While troubleshooting a new HD, I just ran into a new problem of which
I'm not sure where the problem is.

When I press the on/off button for the pc on the front of the case, I see
it start to light up but within a fraction of a second, it shuts down is
the best way I can describe it - does not continue to boot up. When I
press the on/off button again, nothing happens.

When I turn off power completely (using the on/off button on the PSU),
wait awhile, turn back on the PSU power button, and then turn on the pc
using the on/off button on the front of the pc, I get the same problem
described above - pc lights up but immediately stops.

The problem occurred while I was troubleshooting a problem with a new HD.
My testing of the HD involved frequently turning the pc on and off in
order to switch the power cords and SATA cords among the HDDs.

My first thought was that the switch on the front of the pc broke, but
that doesn't seem likely because the pc will power up however briefly
when I press the switch.

Second thought is that there's a "fuse" ?? needing to be reset.

Third thought was wondering if the CMOS battery needs replacing.

Fourth thought was worrying if I need to replace my mobo.

Fifth thought was same question but about my PSU.

Sixth thought is wondering if I corrupted BIOS somehow. I would enter
BIOS to check if the new HD was listed in the SATA info, so I was always
entering and exiting BIOS and resuming booting up.

My notes about the pc are not handy. The mobo is an ASUS AMD mobo about
4 or 5 years old- I don't have its model number handy. The CPU, PSU, DVD/
CD device and graphics card are the same age. The HDDs vary in age, all
SATA. I have four. IIRC, the PSU is rated to handle 600W.

Thanks,

John