If the knocked-out tooth is an adult (or permanent) tooth, your dentist may be able to put it back. You must act quickly. If the tooth is put back in place within 10 minutes, it has a fair chance of taking root again. After 2 hours, the chances are poor.
If the tooth looks clean, put it back in its place (its socket). If this is not possible, or if there’s a chance that the tooth might be swallowed, put it in a container of cold milk. Go to your dentist, or to the nearest dentist, right away. If you get help within ten minutes, there is a fair chance that the tooth will take root again.