Hi!

Thank you ! Well, the mouse and keyboard are not USB - just regular
round green and purple plugs.

These are PS/2 ports. If something was really stuck in a USB port and it
killed the keyboard and mouse too...that's not such a good sign. Since the
system runs, it is very likely that some damage occurred on the motherboard
when the USB port was shorted out.

If the short had caused damage to the +5 volt line coming out of the power
supply your computer probably wouldn't even be able to start up, as many
things depend upon that voltage in order to work.

Look for any breakage or damage to the USB port that might still be causing
a short. You might get lucky--most USB ports that I've worked with are very
well protected against shorting. I've seen some that shorted out and simply
caused the computer to shut down until the short was removed. Others put up
a message stating that the available current from the port had been
exceeded. Remove the short and your computer could recover.

If you cannot find the short, or the port looks to be fine with nothing in
it...then it may be time for either some repair, a new power supply
(unlikely) or a new motherboard/computer.

William