Did you hatch her, get her from a breeder, or a feedstore? How is the ventilation in your coop? Dust in feed or bedding, poor air circulation, and mold from spilled water or wet conditions can promote diseases. There are several common respiratory diseases, and many have similar symptoms. Infectious bronchitis and MG are ones that can cause sneezing and congestion. Are any other chickens showing signs of illness? IB is caused by a virus and won't respond to antibiotics, but you may want to treat her with some, such as oxytetracycline in the water, in cause it is MG, and it might help to prevent secondary infections. You can get that at most feed stores in cattle or chicken medicines.