Hope your hen feels better soon. Poor girl.
Could you share a photo with us?
It could be something as simple as a pollen allergy (I have one hen that sneezes uncontrollably if she goes near certain grass) or maybe some dust in her eye. It could be a bug bite, or a scratch. It could be some deadly super-exhotic new desease that will be named after you and your hen - but probably not this last one.
Usually with this sort of thing, I give it 24 hours before intervening. If it's not immediately life threatening or contagious, then 9 times out of 10, they sort themselves out with a good nights sleep. In the mean time, I would keep a close eye on her and if there were any drastic changes I would take action. Also, if she's being picked on, I might isolate her for a day or two to get her strength up. It may be a good time to check the hens and house for lice and other bugs. If the coop hasn't been cleaned in a while, this might be the motivation to do it.
If I'm really worried about them, but again it's not urgent, I would add electro-vit, or other
chicken vitamin mix to their
water.
If it's still there the second day, or if it suddenly gets worse, then I would do a very close examination to determine why it's happening, and then what to do about it. It could be something as simple as a salt water wash. I hesitate to use an antibiotic cream so close to the eye, unless it's specifically designed for eyes.
Let us know how it goes.