It is very hard to diagnose these issues over the internet without the use of direct physical examination by a professional, and diagnostic testing.
With that being said, I will suggest you focus on broad supportive care which in many cases, will either help maintain her condition or totally improve her condition.
With sick birds, we often immediately predict a decrease in their food intake; among the other problems this can cause, it will affect the bird's ability to maintain normal body temperature.
We should compensate for this by increasing the temperature in the bird's pen, while still watching signs of overheating (panting, wings held away from the body, etc).
Fluid deficits are much more of a concern than food, which is why we should offer warm electrolyte fluids such as dilute Gatorade, or Pedialyte often to help ensure the birds stay hydrated.
This will help to a degree if the bird is still drinking on it's own, but if it's not you should consider tube feeding. The links below provide some information on the topic:
https://www.backyardchickens.com/threads/tube-feeding-ducks-updated-5-7-2020.1211994/
https://www.backyardchickens.com/th...ng-guide-pictures-under-construction.1064392/
A great way to track how much the bird is eating/drinking is by weighing the bird on a gram scale each morning, and by paying attention to the droppings ( scant, watery, dry, etc).
If I had no clue what was causing this, I may treat in a conjecture manner, and start by tube feeding a large amount of activated charcoal, then starting her on antibiotics afterward.
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https://allbirdproducts.com/products/baytril-10