Common things that make a hen stop eating are a blockage of the digestive tract or an infection.
A blockage can be an impacted crop or gizzard or a blockage of the hind gut often caused by a reproductive disorder. You should be able to feel if there is a blockage.... if she is has not eaten for several days her crop should be empty. If it is not, then that would suggest there is a blockage somewhere. Then you need to consider if it feels hard or soft and squishy. Does she also has abdominal swelling? (between her legs and/or below her vent) Check this by cupping your hand and feeling between her legs and below her vent and compare with other healthy chickens..... fluffy feathers can hid a lot of issues as regards swelling, fatty deposits or emaciation. Has she been soiling herself? That usually indicates some abdominal swelling.
What does her poop look like? Is her vent pulsing? Does it look healthy?
Have you checked for egg binding by gently inserting a gloved lubricated finger into her vent about an inch. The oviduct is located slightly upwards but a stuck egg that was causing a problem would usually be just inside the vent, so it should be obvious during an internal exam.
If you have ruled out a blockage of the digestive tract, then it may be an infection. Does she smell bad? Does her poop smell unusually bad?
At this time of year, as they are coming to the end of their laying cycle, their bodies can become depleted of enough calcium to form a shell around developing eggs. This results in a shell less egg and they can be particularly hard for a hen to lay and really make them feel out of sorts. The hen's lower back is usually hunched and her tail down and her vent may pulse or spasm. A calcium supplement given as a tablet or liquid directly into her mouth for a few days may help, if this is the case. Some people use a Tums but a Human calcium tablet with vitamin D or a liquid calcium supplement is preferable or you can buy a special one for chickens.
