Hen keeps one eye closed

[Txfarm2009]

Two days ago I noticed one of my hens had her right eye closed all the time. She will briefly open it but immediately shuts it again. I cannot see anything wrong with it. She really fights if I try to hold her head to look at it. I thought perhaps one of the others pecked her and hit her eye. Or perhaps she got some sand in her eye. But it doesn’t seem to be getting better. She is lethargic and not wanting to eat. She is a 4 year old Buff Orpington who is lower on the pecking order, but usually quite active. Any ideas as to what is wrong???