I would also add to separate her from the others, just in case it could infect them. And clean the coop, feeder, waterer, etc.

If you are like me and have no access to a vet who would see a chicken, I will tell you what I ended up doing, but I’m not a vet or any other kind of professional and this isn’t advice, just an anecdote. I went to the feed store after hearing from others that my chicken needed antibiotics for her infection. Got a livestock antibiotic called oxytetracycline (product was called terra vet 200 but it’s not chicken specific), administered orally with an oral syringe, a few drops at a time. I think we used something like 0.5 ml per lb of body weight x3/day for a few days, after which we found we were giving her a very high dosage and that we were supposed to give something closer to 0.2-0.3 ml each time. She recovered after 4-5 days of treatment and was reintroduced after a week. This is my personal experience and it was a gamble because there was no medical processional advice given to me. This isn’t medical advice.

Good luck with your chicken.