Is it new? Is it in different light than it is used to? Any crabs crawling over it? Was it open 24 hours ago? One day of being closed is no big deal. If it was moved or upset greatly, it may stay closed for a few days. I had some green star polyps that I was sure were goners. Then hours before I intended to pull them out, I saw two or three partially open. Within a week, many were opening and that patch has been growing since. As long as the lighting and water flow are ok, and no creatures are bothering it, it should show within a week. Keep a close eye on it, but don't worry yet.