Have to go along with those prior posters who point out that this kind of thing is NOT The Gig From Hell. When you're simply ignored, it's The Gig From Limbo. The Gig From Hell is when the audience is actively antagonistic. Rock musicians are more willing than folk musicians to play bars, in part because a solid body electric can be used as a defensive weapon more effectively, and with much less chance of damage to the instrument, than an acoustic. That's also one good argument for having a banjo player in your group.

The advice above, to find one person really watching and listening, and play at that person, is also very useful.

Sometimes you can turn it around. It's good to have a few songs that EVERYBODY knows, with singable choruses. Kris and I played a month or so ago for a bus tour group while they were eating dinner. We were completely ignored, as they talked among themselves with their mouths full. As they were finishing up, I did "Home on the Range," and got them all singing. For the rest of the gig, they listened intently, and applauded with enthusiasm, for every song. Of course, it might have been just that they were finished eating. I don't know.

Peter