Can you post some photos of your hen?
What are her symptoms?
Respiratory infection can be caused by a number of things but most commonly disease like Mycoplasma, Infectious Bronchitis, Infectious Coryza and ILT.
With an infection, it would be best to treat symptoms with an antibiotic.
Tylan/Tylosin can be used to treat symptoms of MG.
Sulfa antibiotics are best suited for Infectious Coryza.
You can try VetRx, it's a Remedy, not cure. It may help relieve some symptoms or atleast give a bit of relief like using Vapor Rub does with humans. Once nightly for a few nights. Do make sure your coop have very good ventilation. Not having enough ventilation and ammonia buildup is a another reason for birds having respiratory issues.
https://www.drugs.com/vet/vetrx-poultry-remedy.html
Do keep in mind that respiratory illnesses usually make birds carriers for life. Have you brought in new birds that may have infected your current flock?