It could be a lot of things from some type of injury to a corneal ulcer. I would start with getting some eye antibiotic in there. It sure wouldn't hurt for the vet to look at it and verify what the problem actually is.
I have been nursing a rabbit's eye. He was a rescue. He has a corneal ulcer which probably resulted from the attack that he had endured. He is getting an anti inflammatory drop as well as the antibiotic eye meds.