Separate her from your other chickens immediately for two reasons: so you can make sure she is eating, drinking, pooping, and laying normally, and so she does not get your other chickens sick if she turns out to have a disease.
I can't give you real treatment advice, but I can tell you a few things to do that will not hurt her:
Keep her isolated in a warm dark place.
Give her three drops of Poly-Vi-Sol liquid baby vitamins without iron on her beak per day, tapering off after four or five days. Some chicken problems are vitamin related.
If she is dehydrated, you can give her 50/50 Pedialyte and water.
Make her a probiotic moist mash of layer's pellets, cooked oatmeal, and yogurt, mixed together until soft with a little water. Give her cool scrambled eggs if she will not eat.
Check her vent for signs of lice or mites.
Check her poop for signs of worms, note its color, consistency, and amount.
What are you feeding your chickens? That can help people figure out what might be wrong.
Wiser people will chime in tonight if they can help more.