OK, I'm gonna go a little against the grain here. It is easy to blame the soundman for bad sound. But is that the case here? Your guitarist steps on your cord. Is that the soundman's fault? Your drummer stops cold in the middle of a song. Is that the soundman's fault? I have played hundreds maybe even thousands of gigs. Just about everything that can happen has happened to me. But you never just STOP. Yeah, you might get out of beat for a few seconds, hit the wrong notes or play the wrong chord. It happens (even to the pros). What do you do? You just act like nothing happened and get right back in the groove. Many times people will not even notice, sometimes they even think it's part of the song.
And even you described this song as one of your LOUD ones. Well, when you play a big house it's the soundman's job to control your band's volume. The best sound is when the backline volume is kept down. A good soundman will make sure the house hears you. If you guys had not cranked up so much the drummer would probably have been able to hear the monitors.
I am not saying this soundman was good or bad. He should not have been off with his girlfriend while you were performing. On the other hand, he may have given up on getting your band a good sound. Most soundpeople care about what they do and try to make you sound good. But all too many times bands do not cooperate. They crank up on stage which just ruins and muddles the sound. Then you also have problems hearing yourself on stage.
Next time you play a venue where the sound is provided, go up and meet the soundperson before you go on. Buy the guy a drink. If you have a set list, go over it with him and give him an idea what you are gonna do. But also listen to him (or her). This person knows the venue and knows how to get a good sound. If he or she tells you to bring the volume down on stage, then listen. If you treat this person with disrespect they are gonna get you. And they know how.
If you know something better than Rock and Roll, I'd like to hear it - Jerry Lee Lewis