This is usually caused by a bacterial infection. One of the best things you can do are a couple of water changes, (space them out a few days though, and they don't have to be huge, about 10% is good). Also you want to watch the tank and see if any other fish is harassing the sick one. You may need to separate these fish if this is happening.
Wamp, sometimes if a diseased eye isn't taken care of they will eventually burst or fall out, but they don't just fall out for no reason!