Sorry to hear about your fish. Whatever the reason for the eye loss, it sounds like you may need to euthanize the fish. I certainly would, if it's starving and unable to find food. I believe that the most humane methods are: by netting the fish and slipping it quickly into a small container of nearly frozen tank water, which should kill it within seconds; by placing the fish in a small container of tank water (still at regular temp) and then into the freezer, which some believe is less stressful for the fish even though it's slower; or by transferring the fish to a small container of tank water and using clove oil to anesthetize/kill it.
Clove oil is supposed to be available at pharmacies as a treatment for ear infections or something, but I've never found it for sale and so have always used the freezing-water methods. If I have time to let a container of water nearly freeze, I will use the first method because it kills the fish quickly. I have, when necessary, used the slower method. No process is perfect, so do what you need to do and don't worry overmuch about it.
Again, sorry for your fish.