Dental Emergency Advice: 

Your Crown or bridge has become loose.

Generally speaking, it is not good to re-cement crowns and bridges back by yourself because they might become loose again and you could swallow them but however there are some temporary cementing materials available in the chemist shop to temporary fix your crown back, until you get to the dentist. But before doing this have a good look inside the the crown (or bridge). Is there remaining of the core inside? (a broken core inside the bridge?) then maybe there is not much point to try and re-cement your crown because it has no retention and can comes out. However if you have to fix your crown back temporarily please see the video below :