I apparently shorted my computer when I plugged the output rather than the input plug of a headphone into the headphone jack of my soundcard. It (Dell Optiplex G1, service tag GDY81) turned off and wouldn't turn back on. Only on/off I see is the pushbutton on the front panel, no case-open switch, removing the daughterboard assembly with the game/soundcard and modem didn't help... pushing the on-off button didn't start the pwr sply fan or flash LEDs or anything. And I tried a different cord though the connection to putting the wrong plug in he right hole seems pretty evident.

Now, when I started this message the verbs in the last paragraph were in the present tense ("doesn't" not "didn't") but just now when I went to check if there was voltage to the front panel pushbutton the machine turned on. So my problem is solved but I'm still curious: Is there a time delay in place of a fuse? Is there a capacitor I could have discharged to shorten the wait?