Hello happychickens! Could do with a little more information
Has your hen got any other symptoms -like is that side of her face hot compared to the other? This could indicate a local infection likely cause trauma by pecking or catching on something sharp whilst foraging.
Chickens are also prone to upper rispiratory infections -mycoplasma being the most common particulary in young birds. Odd that only one eye affected. Is your hen eating/drinking pooing as normal? Does she appear hunched with fluffed feathers? Can hear any wheezing sounds. Look closely at her beak and you will see if she is gasping at all.
With either of these symtoms I would take to a vet and ask for antibiotics.
In the meantime a good all round tonic is apple cider vinegar (from feed stores) and pop some crushed garlic in their water. Hope this helps!