What does a doc do if one of your permanent teeth falls out?

Replace it: A tooth "falling out" generally means there is a bigger underlying issue. The entire tooth, root and all, indicates an advanced gum infection or worse. The top half, called the crown, means there is underlying cavity or fracture. Remember that the mouth is like a puzzle, and when a piece is lost, then you no longer have a puzzle that fits. Seek a dentist.

Options: Replacing it is best. Implants are usually the best option but costly initially. Their longevity makes them less expensive in the long run though. You may use a bridge, or a removable appliance. All are good options you should discuss with your dentist.