You must visit your dentist immediately. The dentist will probably take and x-ray examination and then place medicated cement over the sensitive part of the fractured tooth. If it is a serious fracture, he may place a band or a crown over the tooth to protect it. If the tooth is knocked out, try and rinse it in water and try and place it in the socket from where it has detached, in case that does not fit well, wrap it in a wet cloth or place it in milk. Go to a dentist as early as possible since in many cases it is possible to re-attach the tooth to the jaw so that it functions normally.