sick chameleons almost always close both eyes. If it's one eye, then there's simply something wrong it that one eye - usually just an irritation. I see it more in cup fed animals than others. There is usually much more supplementation on cup fed insects than others, and when they are caught, they almost always fling lots of excess supplementation towards the chameleon. They close their eyes reflexivly, but often, the insects still have a lot of powder on them, and it does get in their eyes.
My male veiled had his left eye shut for 3 days this past week. I just flushed it every day, and now it's fine.
We were away for the weekend, so I filled his bowl with some superworms (normally I hand feed him). When I got back, his eye was shut. Probably got some dust in it.