Not enough info to really pinpoint the cause.
If it's dropping ALL audio to both mains and monitor mixes, it's unlikely that ALL amps are doing the same thing at the same time, unless that is, you are running into a power situation. Even then, amps would come on and off at slightly different times and you'd have visual cues that there's a power issue.
The next clear thing to know is how are things routed? Are you using stage boxes over AES50, or connecting right directly in and out of the board? That connection could be a source of complete failure.
When there is a failure, is there any indication on the mixer itself? Do you still see meter levels? Do you see any strange behavior?
Lastly, is this connected to a network? Is that network SECURE?? If you have an open WIFI network, or an easy to guess password, anyone with a phone in their pocket could potentially mess with your show.
