When an existing crown comes off it can sometimes just be re-cemented to restore. The most common problem is that some of the tooth breaks off inside the crown and there isn't enough tooth structure to hold onto. If tooth does break off inside a crown, the tooth can be removed from inside and hopefully re-cemented. A build up can be placed on a tooth to provide strength by building it up for more tooth structure. A post and core can be placed if the tooth has had a root canal for even a stronger base. Worse case scenario the tooth would be too broken down and the tooth would have to be extracted.
Dr. James L. Johnson