A chicken so depressed she's got her head on the floor is probably not going to get better on her own, in fact, she's probably close to checking out. She needs your immediate support to have much chance of survival. Things you can do tonight: purchase Pedialyte at the grocery store and give her that instead of water. Give it by eyedropper if she won't drink on her own. Also purchase a small carton of plain yogurt. Mix 1/3 cup of her feed with enough Pedialyte to make a stiff mush. Add 2 Tsp. of plain yogurt. Again, if she won't eat on her own, you'll need to hand feed her. If you have her quarantined where you can observe her for other symptoms, all the better. Just a stab in the dark...it's probably Coccidiosis and just because this bird is the only one showing gross symptoms doesn't mean the rest of your birds aren't infected. This is just about the right age for cocci even if your birds have been on medicated feed. If you'd rather wait and watch and hope that you can come up with a firm diagnosis, that's fine. But a bird in this condition won't survive long without food and water.