What happened was you prolly blew something up good. The outputs from your PA are sending a low impedance signal of however many watts, (50-1000 Somewhere between there I'm guessing.) You sent that to your Speaker 1, good, which then sent it to Speaker 2, that's good too, but ugh oh... you then sent it to the computer in. The computer in is most likely looking for a high impedence signal from a CD player or keyboard or microphone or guitar, not an amplified signal from a power amp. So you sent 50-1000Watts of power into a reciprocal expecting about <.1watts. That's bad when that happens.
The best way to do this is run from the microphone into your computer via soundcard, then into the PA then out the speakers.
The next best would be Microphone into a channel on the PA, send that signal out an AUX to the computer, send the computer signal into the PA and then out to the speakers.
There's no need to go back to the computer from the speakers. If you want to record this performance (there's a few ways of doing it,) mic up the speakers, send that mic directly to the computer but make sure where ever it's getting recorded to, isn't assigned to an output.
Tough luck dood. I'm guessing somethin's dead though.
