How long has he had the star?
What color is it?
How old is the tank?
What does he feed it?
What are the water paremeters, especially pH, salinity?
Any water changes recently?
Did it lose any arms?
This can be caused, usually depending on the species, by eating too large a meal, or by osmotic shock. It is important to know more about what happened. Typically, they can easily recover from eating injuries, and often from osmotic shock injury, but not always. Usually it is best to leave it in the tank (do not remove to another tank). Remove anything that might be picking at it (or place the brittle in a critter cage in the tank). Try offering food.