Hello, this is Dr. Linda. I hope I can help with your question today.
I am very sorry for your concerns about your hen. This is, unfortunately, a pretty common occurrence when a bird becomes unwell. All the other birds can become quite aggressive.
If you have not already done so, please isolate her from the other birds. You can either fence her off from the others, or bring her indoors.
Then, you need to decide whether to have her seen by your veterinarian, or try to treat her at home. Your veterinarian can check her for intestinal parasites, or determine if she might be egg bound, or if she has already developed a condition called egg peritonitis. This is a situation where basically the egg contents have been deposited in the abdomen & have now become a source of infection.
If you want to try treating at home, you will not have the benefit of antibiotics, but sometimes just good TLC is helpful. I would deworm her with pyrantal pamoate. For an average size hen, all you need is about 1/2 cc so I would see if your veterinarian will sell you this small amount. Otherwise, you can purchase this under several brand names at any pet store. You can also purchase flock dewormers, too.
Personally, I like to bring all my sick hens indoors. This will keep them a little warmer, & certainly cleaner. Then offer all the regular feeds you normally use, plus other nutritious items such as meal worms ( my hens go crazy for them & you can buy them by the bagful at most garden & feed stores, & even Wal Mart ), cooked egg, cornbread, & even pancakes.
With time, she may turn around if she is not too sick with something as serious as the egg peritonitis. Even with medical help, this is a unrewarding problem to solve.
I hope this gives you some direction to try & help your hen. I have one right now that I am trying to get well, & after a week, she seems to be doing better. Have patience.
If you have any further questions or concerns, please do not hesitate to let me know. I will try to help as much as I can.