If it's quiet and not moving around much, I agree with the sugar water:  they get low blood sugar, and it knocks them for a loop.  I'm also a big fan of "mush" made from chick starter crumble or mash, and water.  If you scoop some up on a finger and say, "Chick chick chick!" then the chicks think you're Mommy showing them a treat, and they will peck at it.  Sometimes it can take five minutes for the sugar water to kick in, so give the quiet one about 5 drops, wait a few minutes, then offer the rest some mush.  Hopefully when the quiet one sees the rest getting all excited, it will want some too.  A super boost for chicks is a small amount of hard-boiled egg yolk--maybe half the size of a pencil eraser, per bird.  It doesn't take much, and it's packed with nutrients.  They love it.
If it has diarrhea (pasty/dirty butt) then electrolytes are also a good idea, but generally if there is no diarrhea then electrolytes aren't essential.  Good luck!