I don't know what kind of salvia you have, but my perennial salvia basically takes care of itself. I usually deadhead the blooms, and the plant will rebloom again later. Since yours have fallen off, your plant may be different than mine, so I'm not sure there is anything you can do to promote reblooming. I believe mine is May Night.

I use a hedge shears and cut off quite a bit. They get thicker and rebloom again.