Hello there and welcome to BYC!
Sorry about your bird. Chickens will attack another bird if this bird is sick. It is there way of protecting the flock from illness. So in their minds, it is time to run this one off.
Have you wormed your flock recently? Worms will destroy your birds and and kill them. It usually starts with one bird and then eventually all the birds one by by succumb to worms. So get her wormed if she has not been wormed in a while.
Check for egg binding. Stop by this article on egg binding in if she is egg bound, there are helpful hints here on getting the egg to move... https://www.backyardchickens.com/a/egg-binding-symptoms-treatment-and-prevention
Feel her belly, between her legs on the outside. Is it soft and squishy or firm and small like the others? Soft and squishy could be from internal laying or fluids leaking into the abdomen.
Check her crop first thing in the morning. It should be completely empty and check it at roosting time to make sure she is eating. It should be completely full.
For now, separate her out and cage her if you have to, to make sure she is eating and drinking.
Any other symptoms she is exhibiting? If you think she is becoming sick and have an Avian vet you can take her too, you might try that. But you should also post this in our emergency section if you feel she is ill. Lots of knowledgeable members there as well that can help you...https://www.backyardchickens.com/f/10/emergencies-diseases-injuries-and-cures
Good luck and I hope you can get her back to her old self soon.

Sorry about your bird. Chickens will attack another bird if this bird is sick. It is there way of protecting the flock from illness. So in their minds, it is time to run this one off.
Have you wormed your flock recently? Worms will destroy your birds and and kill them. It usually starts with one bird and then eventually all the birds one by by succumb to worms. So get her wormed if she has not been wormed in a while.
Check for egg binding. Stop by this article on egg binding in if she is egg bound, there are helpful hints here on getting the egg to move... https://www.backyardchickens.com/a/egg-binding-symptoms-treatment-and-prevention
Feel her belly, between her legs on the outside. Is it soft and squishy or firm and small like the others? Soft and squishy could be from internal laying or fluids leaking into the abdomen.
Check her crop first thing in the morning. It should be completely empty and check it at roosting time to make sure she is eating. It should be completely full.
For now, separate her out and cage her if you have to, to make sure she is eating and drinking.
Any other symptoms she is exhibiting? If you think she is becoming sick and have an Avian vet you can take her too, you might try that. But you should also post this in our emergency section if you feel she is ill. Lots of knowledgeable members there as well that can help you...https://www.backyardchickens.com/f/10/emergencies-diseases-injuries-and-cures
Good luck and I hope you can get her back to her old self soon.