How old are the chicks? We had newly hatched ones that did a similar sad thing and they did not survive. The little chicks are delicate, they seem to go down fast. The basics I was told is to make sure they are drinkng water. Try electrolytes (pedialyte is what we tried) with a dropper...be careful not to drop too much, they can aspirate on the liquid. You may want to look that up on the forum, because I am not sure how to be sure the liquid is going down the right plumbing. Whenever it got this serious with new chicks, we did not have a good outcome. Do you have the right temp? No drafts? Are there other chicks doing well? Is the sick one being trampled?
Just a few thoughts. I wish you luck.