Firstly, it may be an idea to isolate the sick chicken. You could try antibiotics and vitamin supplements - the latter helps boost the immune system. It may be a mild respiratory disorder, or could be something more serious. My flock has infectious bronchitis (same symptoms you describe) which is a chronic condition (i.e. not serious, but all the flock are carriers and they show symptoms now and again). As i am relatively confident regarding their symptoms i don't do anything apart from ensure vitamin supplements are in their water. I do monitor them a few times a day, but the symptoms usually clear up after a week or so. A bit like a human having a cold.
However, since you do not know what is going on, antibiotics may assist - either in fighting whatever "nasty" she has or at least fighting off secondary infections if it is viral (e.g. infectious bronchitis).