If a tooth has been knocked out as a whole - without being fractured - your dentist can sometimes reposition the tooth in the jaw bone. This is the optimal solution. Knocked-out primary teeth should not be reinserted, according to a majority of relevant literature
The success of reinsertion depends largely on how long the tooth resides outside of your mouth or transportation medium. The tooth should be reinserted as quickly as possible. It is strongly recommended that this is done by a dentist only.
The ideal transportation medium is a tooth rescue box. These are available at most schools, but otherwise they are rarely readily available. Otherwise long-life milk is a good option. Also make sure not to touch the root.
Also note that punches to the face that cause the loss of a tooth often also injure bones and soft tissue. In case of graver injuries, you should also consult an oral/maxillofacial surgeon at a clinic.