I would try these things, if I were you.
1. See at vet. He/she can definitely give you a diagnosis on what's wrong with your chicken. The sooner the better.
2. I would get Emeraid's avian nutrition formula. A prescription is required from your vet, if he/she doesn't have it in stock. I would give it to her via a plastic syringe (amazon) and feed her. We once had a chicken with gout who wouldn't eat, but gave her the nutrition formula to give her calories and make her stronger, and she responded and was eating on her own within a few days.
3. Get Elector PSP for chicken mites. It's harmless to humans and chickens, but it works really well.
These are the syringes we used:
https://www.amazon.com/gp/product/B09FZ4ZF1R/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&psc=1
We feed her morning and night one full syringe (1 1/2 syringes if she can eat it). Mix the formula in water so that it isn't too thick or too thin. So that the syringe can pick it up.
Here is link to Emeraid formula:
https://www.vetrxdirect.com/product/view/emeraid-intensive-care-diet-for-omnivores-rx
Hope your vet has it in stock since I think time is of the essence. If not, you can put an order in and have your vet send them the prescription.
Chickens can be purchased for a few bucks, but they steal your heart. We did (and are doing) a lot to keep our chickens healthy and happy. Your birds mostly likely have Marek's since most backyard chickens have it (heard from our vet). I'm not sure if this is the issue, but your vet can give your a definitive diagnosis.