Have you just broken your front tooth opening a bottle of beer? Or bit on a hard nut on your back tooth?
DO NOT PANIC!
Fractured teeth can be repaired at your dentist. The treatment will depend on how much tooth is left. Sometimes it is as simple as bonding white filling material (composite) to your tooth. Sometimes, more extensive treatment is required. If you have the piece of missing tooth, keep it in water and bring it to your dentist. In the meantime, avoid anything too hot or too cold.