I just lost the long reply I wrote. I need to go do something else at the moment, but I will be back soon. Your hen isn't sick. I believe she's the victim of bullying and has been chased from food and water and is now weak from starvation. This mimics serious disease.
Feed her cooked egg and...
Did you see my post about asking at Tractor Supply for amoxycillin?
A bacterial infection can occur at any time for any reason. Often some stress or exposure to higher than normal bacteria in the environment can overwhelm a weak immune system. I've had hens get sick with a bacterial infection...
If you are in the US, youmay be able to find amoxycillin at Tractor Supply. It might say it's for fish. It's what most of us use for chickens.
The dose is 250mg per day for ten days.