Photos would help. Many of us are experienced chicken keepers and we've developed a "sense" when glancing at our chickens. We are able to spot a chicken that is sick and by the various "body language", we can tell if the chicken is in pain, suffering from a crop problem, has liver disease, has gotten into something toxic, or has a respiratory or sinus infection. Yep. All by looking at the chicken.
You can also help us by telling us what your chicken's daily habits involve, where she spends her days, how many other chickens you have and their approximate age and if any of them are recent additions. Also, what has your weather been like lately?
Absent this information, I will guess this hen is suffering from coccidiosis or a bacterial infection. It will do no harm to begin treating her and the entire flock for coccidiosis. Ask for Corid at the feed store if you live in the US. The liquid form is mixed at a ratio of two teaspoons to a gallon of fresh plain water, mixed fresh daily. You will provide it as the only water source for five days. Repeat the treatment in one week. Do not give vitamins during treatment.
The sick chicken will need to be given her doses at regular intervals with a syringe if she won't drink on her own.
If this doesn't cause improvement in your little girl within 36 hours, I recommend starting her on an antibiotic.