I've saved a few plants over the years by killing the borer (a razor blade works well) and then mounding up soil over the bottom of the stems and keeping it moist for a couple of weeks. Sometimes it works, sometimes it doesn't. I suspect it depends on how much damage the borer has done before it's dispatched. It's certainly worth a try.

Also, it's not too late to try for another crop. Seeds planted now might make a few squash before frost. They germinate at record speed this time of year if you keep them damp.

Sandy's right.....plant some more seeds...
I had to do it yesterday, borers killed my plant.