I have had some hens like this in the past. I gave them yogurt, garlic, apple cider vinegar, turmeric, a pinch of cayenne, and a little cat food (protein source) that I ran through the blender. I dipped their beak in it to get her to eat some of it, and then I isolated her. She was fine after a few days. This has always worked for me when I have had the odd chicken not feeling well. Now if your hen has some kind of egg related problem, then this will most likely not help any. I have not had many chickens with those type of problems, so I will not be able to help you there. Good luck! I hope she gets better soon.
Good info! Yes, defiantly give her garlic and yogurt. Don't give ACV until we are sure of what she has. Avoid treats such as mealworms, scratch, etc. Oatmeal and boiled egg are good though.