I am not a expert on diseases but separate the chick until you know what is wrong, you dont want the rest of them to catch whatever it is!
How old is the chick? Do you have any vitamins with electrolytes, Pedialyte, Or Gatorade that you can feed her? Is she hunched or puffed up, or having diarrhea or blood in the poop? Can you get some Corid (amprollium, Ampromed) from your feed store today? Coccidiosis can be very common in chickens 3-20 weeks old. If she is younger than that she could be dehydrated. Here is a good link tor read about cocci: