Greetings, I am Dr. Pat. I have worked with birds for many years. I will do my best to help you.
If she has been laying, I would worry first about calcium deficit, egg peritonitis or other reproductive issues. If it has been very cold, that can exacerbate many illnesses and complicate matters. If the water is cold, they often will not drink it and then there is dehydration on top of everything else. These of course are just guesses, and there could be 100 other possibilities.
You can examine
her thoroughly again, including opening the mouth and having a good look in there for mucus, redness, masses or anything else unusual. You can take her temperature gently with a rectal thermometer. Anything above 105F/40C is significant. Palpate the tummy for an egg, fluid, lumps or anything else. Check all her joints for swelling, pain, and mobility. Crop the crop to see if it is empty or packed full.
Move the bird to an aquarium, box or carrier with soft towels or hay in the bottom, no perch, and food and water in low bowls that she can reach easily. Put the whole thing on a heating pad on low or medium. Keep her partially covered, warm and quiet.
Do not try to force food or water. If she is readily taking water, you can make sugar water. If there is a pharmacy open in your area, call and ask the pharmacist if they will sell you calcium glubionate syrup without a prescription; she will need about 2cc twice a day. It is not perfect, she needs injections if she is calcioum deficient, but it may be hard to find a vet today. You can offer warm cooked rice, pancakes, cornbread, grapes, melon, greens in addition to normal food. Transport as soon as possible.
She needs to see an avian/poultry-experienced
veterinarian ASAP for complete examination, diagnosis and appropriate treatment. Check this link
for members of AAV in your area or call your regular vet and see who they recommend; ask if they really have worked with birds a lot and especially inquire as to poultry experience. You can also check here
for an unofficial list.The expense for this is going to be a lot less than inefficient, ineffective, dangerous treatments, guesswork, and loss of the flock; not to mention possible implications to human consumption of tainted eggs. Many states/governments have poultry diagnostic labs that charge very reasonable fees to test for common diseases. If you have others and presumably use the eggs, it is important to have a solid diagnosis and treatment safe for egg consumption.
If she were my patient, I would start with complete fecal analysis and direct smear, for multiple parasites; bacterial culture and sensitivity of the feces and choana. Depending on the case I might do a fungal culture. Routine blood work is necessary to rule out other issues. I would very likely order a number of DNA tests for poultry viruses as well. Generally I start them out on antibiotics as indicated by the tests (I use a lot of human antibiotics that are injectable).
Pet/feed store medications and home remedies are harmful, ineffective, immuno-suppressive, and make them much worse and may interfere with the veterinarian's diagnosis and treatment. Do not use them.
The flock should be on a high quality pelleted laying hen diet with extra greens/pasturage. Overcrowding, cleanliness, proper water, environmental temperature, humidity, ventilation, photoperiod, and toxic exposures should be addressed. Check this link
for husbandry advice. Check this
for basic care, set up and maintenance.
She may need injectable antibiotics, calcium and many other medications. Act quickly and good luck.