One word: Relax.
It stinks......but sometimes it doesn't go well. You wouldn't quit because of one bad practice... don't quit because of one bad gig.
Sometimes just the bad gig will make the next one better because everyone will be so determined not to mess up again that they pay attention. Mistakes can snowball if you let them ("shoot, I missed that note.......now I missed that one...I'm an idiot...."). Trust me....I make plenty of them. Used to throw me for a loop and I was off the rest of the night, but then I realized that no one else in the audience had a clue as long as I didn't make the *mistake face*.
If band seems discouraged....point out to them that unless you guys look miserable 99% of the audience isn't going to have a clue. Guitarist doesn't have his perfect tone? pffffff who notices besides him. Etc....
Make sure your singer has plenty of water and doesn't strain themselves, we can restring when strings go bad but a voice takes a bit more to fix!
You'll be fine tonight......may I suggest getting there early enough to fix PA problems so they don't ruin the night
