Hi XXXXXXXX,
You sound like a knowledgeable pet owner and you know that your hen may be very sick. Birds are notorious for hiding their symptoms of illness until they are critically ill. You should get on the phone right away to your avian vet even though the clinic is closed and see if they have an answering machine where you can leave a message, of if the machine tells you another number to call. If necessary call several other local vet clinics and see if they know of a vet who will see birds after hours or an emergency clinic that will see birds. You can also look for an avian vet at http://www.aav.org/, and one of them might be available to see you over the weekend.
Medications you can get from pet stores or feed stores are rarely effective, but if you can't find a vet to see her, you could start her on an antibiotic. This will interfere with some test results, but if you feel you have to do something this does have at least a small chance of helping. Bacterial infections are the most common cause of illness in birds, but the pet store medications have been around for many years and have become ineffective against many bacteria, which is why we veterinarians usually don't recommend them.
You have done exactly the right thing by bringing the bird inside away from your other birds. Chickens will often attack a sick, weak member of the flock, and may even kill her.. Extra heat is almost always helpful for sick birds, so keep her at 80 to 85 degrees if possible. Don't leave her in the dark all the time because she won't eat and drink, so give her light for 12 to 14 hours.
Best of luck with her, and feel free to email back with any more information you can give me.
Avian Veterinarian
21 years experience with birds; state representative for Association of Avian Veterinarians