Basically, the crown needs to be cemented back in place (if viable) as soon as possible. We recommend you contact the practice immediately to obtain further advice and an appointment. When a crown is loose or comes out, several things may take place all to the detriment of the tooth, so the sooner the status quo is re-established the better. Some of the things that may happen are:
Some crowns come off cleanly and are really simple to clean up and re-cement.
Some crowns are placed with a stronger adhesive and may need to be cleaned by a laboratory.
Some crowns have cores and/or posts to anchor them and these can be broken too!
So the sooner we see you the better.