If your crown came off intact
Save the crown — rinse it gently and place it in a small container. Often we can re-cement the same crown if the underlying tooth is healthy. Don’t try to glue it back on with anything from the drugstore.
If a filling fell out
The exposed tooth may be sensitive to hot, cold, and sweet. Avoid chewing on that side until you can come in. Replacement is usually a single-visit appointment.