Did your dental crown just pan out?
The first thing you should do is to get the crown out of your mouth. By doing this you minimize the risk of swallowing or accidentally breathing your crown in. If you end up accidentally breathing your crown in, it could cause infection and further problems.
The next most important thing to do when a crown comes off is for you to call an emergency dentist and let the office staff and let them know the urgency of what happened. Give them as much details as you can and schedule an appointment at the soonest time possible.