The stomach or intestines may be injured by heavy blows to the abdomen. Bruising these organs can cause bleeding in their lining. See a doctor immediately if you suspect internal bleeding.
A blow to the solar plexus, which is a nerve center in the abdomen, may short-circuit the nervous system causing a knockout. Usually, nerve function is restored within a few minutes. If this happens to you while sparring, rest until you feel normal and then pick up where you left off.