I'm sorry to hear of this with your hen. She sounds to be terribly ill. In any case of respiratory illness in chickens, it's important to know if you're dealing with a viral, bacterial, fungal, or parasitic disease. The treatment for one disease may be ineffective or even harmful for others. Outward signs may appear similar to the flock owner. To make a diagnosis, your avian-oriented vet (please see here: www.aav.org) or county-extension poultry personnel can perform several tests including bacterial cultures of the airways, blood tests, and necropsies (post-mortem exams) of dead birds if they are available (refrigerate - not freeze - their bodies until they can be brought to the vet or your county health officer).
Presumptive therapy with Duramycin-10 (tetracycline) available in feed/agriculture merchant stores is appropriate for affected flocks. A single bird is better addressed with an antibiotic prescribed by your vet that you could administer directly into the beak or by injection into a breast muscle.
Small poultry flocks are susceptible to a number of respiratory infections. Some of these produce extremely mild illness while others may result in a high number of deaths. Regardless of whether birds are raised for meat, eggs, breeding or show purposes, respiratory infections result in decreased performance. Common bacterial diseases include fowl cholera (Pasteurellosis), chicken coryza (Haemophilus paragallinarum) and avian mycoplasmosis (Mycoplasma gallisepticum); infectious bronchitis is a common corona viral infection.
It's best to approach the diagnostic process with a clear sense of the bird's financial value to your operation. Although some services might be available free of charge through a county agency or land-grant extension office, the expense of some diagnostic tests and treatments can add up quickly. While it’s always worth your time and money to identify a bacterial or viral infection that could potentially impact more than one member of the flock, this might not be the case with a condition that only affects one bird.
Until she can be attended to by a vet, please bring her inside into a "quarantine area", provide her with a comfortable and warm (85F) environment and offer her anything she'll eat - pancakes, cornbread, the tops of fresh greens, diary products such as yogurt and cheese, mealworms or tiny pieces of meat, fruits such as apples, pears, melon, kiwi, and berries, vegetables such as broccoli, carrots, beets, asparagus, cabbage, sweet potato, and squash, and even tiny pieces of meat. Please respond with further questions or concerns if you wish.