sudden death???

[sea gnome]

My coral was doing great yesterday am and when I next saw it 12 hrs later it looks dead, all the others are fine and chemistry isn't off. It is a turbinaria pellata (one of the cup corals). I don't have picture but it has had no stresses that I can see and has no polyp extension at all. In fact it has holes where polyps shoud be. Would the sailfin tang be a suspect, I have never seen it picking at it? Rachel