If a fish doesn't eat, it may be an outset of a disease. Or the fish may want a change of food. Do you give it the same kind of food all the time? If so, you could try some other kind of food that star fish will eat (I personally have no idea what star fish eat) and then, once it starts eating, you could come back to your old food. You say that it doesn't move at all. Did you check if it's alive at all? I didn't even know people keep star fish. What kind of aquarium is it? Saltwater?