It is certainly hard to find a problem if you are unable to recreate it. It may sound obvious, but start with that. Think back to the times it happened... does anything stick out to you that may have caused it? If not, play some music and start jiggling inputs and outputs between the console and the amps - it could very well be a bad cable.
Is the system stereo or mono? Do you have delay speakers or front or side fills? Do you have a video feed and if so did you lose audio to there or to any of the other speakers? If the answer is yes to any of these then find where those feeds are tapped in and you know it's somewhere between there and the console.
Another thing... Is it a possibility that you just straight up lost power to your amps or DSP? Was there a pop of any sort when it all came back on?
