Depleted batteries in a properly set up wireless mic simply make it go silent.
If the receiver has a squelch control and it's incorrectly adjusted you'll get a very loud hiss (like escaping steam or you're standing next to a waterfall.)

The mics have been mentioned, but they aren't necessarily to blame. There was a guitarist mentioned, so there were probably some other instruments too. It could have been any amplified instrument, the mixer or the power amp. Electronics, power supplies, loose connections... almost anything.

"it's funny that the thing worked perfectly for three hours prior. "
What's more disconcerting is that it carried on working normally afterwards, so it's going to be hard to find out what the fault was.

The worst interference I remember while playing in a band was in a place in the 1970s that had a "one armed bandit" gambling machine in the bar outside the back of the hall. The clunk-clunk-clunk of a payout was accompanied by a simutaneous series of loud cracks from the speakers of our PA.
Proper balanced mic cables and inputs were rare in those days, and cheap home-made PA systems (like ours) with 1/4 inch jacks were commonplace.