Why did the vet think not CNN?
Hopefully she's had antibiotics and B-vits? Maybe steroids too? That cocktail will treat pretty much anything it's likely to be.
We did have CNN on the moorland farm, and what you describe would be a fairly common instance of it. As far as I can remember, all the ewes we treated recovered. CNN is a neurological condition, but if it's something else, we found that sheep's eyes have tremendous regenerative capability (*), so
for your ewe.
(*) We had a ewe lost most of one eyeball to maggots. She regrew that eye, looked exactly the same as the other eye. Eventually she got some sight back in that eye too.