First, find the tooth. Immediately after, call your dentist. Then wrap the tooth in clean wet gauze. It's important not to clean the tooth. Cleaning can damage the attachments on the tooth. If the dentist determines that the socket is not damaged, he or she will try to re-implant the tooth. This is best done within two hours after the accident.
While there are no guarantees that re-implantation will work, if the tooth and socket are not damaged, it is definitely worth the effort and many cases result in successful reattachment. Your dentist will need to follow the progress of the situation for many months afterwards, as in some cases, additional treatment will be necessary.