In a situation like this, it is easy to panic, but try to stay calm and composed. 

First, find the tooth. Hold the tooth by the crown (the top part of the tooth that sticks out of the gum) and not the roots. Rinse the tooth in milk or saline (salt water). If it is an adult tooth, put it back into position. Place a thick gauze or handkerchief over the top of the tooth and bite down gently to keep it in place. 

Seek immediate dental attention. The longer a tooth stays out of the mouth, the less chance there is to save it. If the tooth knocked out is a baby tooth, keep it in milk – do not try to replace it with the gum socket. The child will need to be assessed by a dentist or oral hygienist to check for damage to the mouth or gums.