A sharp blow to the abdomen can cause a spasm of the diaphragm. The diaphragm is a dome-shaped muscle that separates the abdominal cavity from the thorax. It is the key muscle in the respiration system. When it flattens, the pressure in the lungs reduces and air flows in. When in spasm (like a cramp, triggered by a sudden knock), it makes it hard to breathe for a few moments; this is the sensation of feeling winded. This will relieve itself rapidly if the person keeps calm and lets the spasm run its course.
Answered by Aneel Bhangu.