I also had them fall off their perch into the sand and they didn't like that. I thought they were gonners. Picked them up, gave them a little current, and they were happy in about two weeks.
The algae on the exposed skeleton leads me to believe it has been a slow demise. Maybe you need to feed a little more, provide a little more light, or alter the flow across the polyps.
These are all shots in the dark since I don't know your tank.
Hope it helps!