If the tooth is clean and if you can, try to put the tooth back in. Hold the tooth by the crown and place it firmly back into the socket. Bite on a clean handkerchief for 15-20 minutes.
If you cannot get the tooth back in, it has more chance of survival if you can keep the tooth in your cheek until you can get to an emergency dentist. If this is not possible, keep the tooth in some milk.
The tooth needs to be replaced ideally within 30 minutes, so seek dental attention promptly.
More information:
Knocked out teeth