It is important to quickly attend to a loose or broken filling or crown, because once a filling or crown has lost its seal, a cavity will quickly form.  Also, a cavity in the area of a restoration is already past the first line of defense of the tooth.   Waiting beyond a few days may allow the decay to reach the nerve.  Should this occur, the only treatment option is a root canal.  An additional risk of waiting is that the filling or crown might fall completely out of your tooth during the night.  If it enters your windpipe, you could end up in surgery at the hospital.