I was working on my friend's PC when it started making an alarm type sound. It was a low pitch then a high pitch, alternating every second (much like one of the typical car alarm sequences). Has anyone ever heard this sound before? Anyway, I instinctively pressed the Power button. I then realized that it was an ATX power supply that uses a "soft power on/off". When I tried to turn it back on w/ the power button, nothing happened. No fan, no front panel lights, nada. I tried everything: different outlets, clearing the CMOS by switching jumper settings, reseating memory, etc. Since my friend bought it used, I don't have much info on the PC. I guess I should have looked for some identifying numbers on the motherboard, but I didn't. I'm taking any suggestions for when I go back there today or tomorrow.

On some motherboards that alarm indicates a failure of the CPU cooling fan, and they will not power on again until the condition is fixed. At this point you will probably want to try to figure out what board it is, because you will want the manual for it. Then you would want to at least replace the fan on the heatsink, if not the entire heatsink/fan combo.

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mjc
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Celts are the men that heaven made mad, For all their battles are merry and their songs are all sad.

The PC might also make an "alarm sound" if it is not able to detect any memory, or if it can't detect memory in the first slot. While working on the PC, did you jar any of the memory loose? Depending on what you were doing, remove and reseat the memory to make sure it is snapped in place.

regarding the lack of power:

Try holding down the power button for 6 seconds, release, and then hit the button again like you normally would to turn the PC on.

Could you have accidently knocked off where the soft power button connects to the motherboard?

Although unlikely, the actual soft power switch could be bad or possibly not functioning correctly due to how the switch is aligned with the front cover. If the front cover is not on correctly, you may not be able to push the "power" button all the way. In any case, you can take a regular screwdriver and short the two pins that the soft power switch connects to. You can use this method (although not preferable) to turn the PC on and off.