Is she looking like she is starting to molt? Are you seeing loose feathers inside the coop? Has she stopped laying eggs? What does her crop feel like? Can you try to get her drinking water, and offer a little scrambled egg bits or tuna, along with her chicken feed?
Try to offer some scrambled egg, tuna, or canned cat food which many chickens find tasty. But then offer her normal chicken feed for most of her diet. Water is more important than food, and it can be added to a small bowl of chicken feed.
Do you have any pictures of how she is appearing to you? What do her poops look like (pictures welcome.) Does she appear hunched or puffed up, and is her tail up or down?
I just went through the same thing. Unfortunately my DaphneDuck passed Sunday night. Like yours, really no symptoms. For only a handful of times over about one month, she wasn't wanting to eat mealworm treats in the evening, which is never passed up on, & just not seeming her usual self. She would stand around watching my other 8 eat treats and/or go sit down. I did see her eat food, she was drinking plenty, & pooping. Most days she seemed completely normal. Even the same day before she died, she was out swimming, foraging around with all the others. That evening, tho, she went into the pen, no mealwoms & soon after went into a dog igloo that I have in there. She did come back out a bit later, but that is ultimately where I found her in the morning. I could just feel that something was wrong, so I was going call Monday morning & get appt with vet.
I'm worried about my others in case of something contagious. I've been doing a lot of research but with no symptoms to go on it could be anything. I did read, however, about someone who had a similar situation and the vet prescribed a dewormer & antibiotics, but I don't know the outcome of any test results, but duck did seem to be doing better just a day later. I'm hesitant to treat everyone tho when it may not be necessary.
I will keep you posted if I learn anything new.
Some things to consider would be a reproductive infection, possible coccidiosis an intestinal infection from parasites, worms, or something else. Corid in the water will treat coccidiosis. Valbazen is very good to treat worms. Amoxicillin (Fish Mox) is a common antibiotic that may be used for reproductive infections, and can be found sometimes in feed stores or online.
I gave her a scrambled egg, which she attacked vigorously. She is ver fluffed up in the morning, but this was her just a few minutes ago. Maybe it’s just the cold, but I’m going to deworm the flock because I’ve never done it so that’s a definite possibility.