A lost crown is generally not a serious emergency, but if you still have it, you should get it recemented. The sooner you do this the better. The absence of a protective crown can lead to increased sensitivity and pain.
If you were able to find the missing crown, store it in a safe place until you are able to see the dentist. You may also try to reapply the crown using a store-bought denture adhesive, usually thinned with water. If it seems to stay in securely , this is still a very temporary solution; you don't want to swallow it (don't chew on it).
Though only a temporary solution, this can provide protection to your tooth and limit sensitivity and pain until your next appointment. Meanwhile, if you can brush it comfortably do so, making sure you do this without toothpaste, which can irritate it.