The medical term for it is phrenospasm, but we usually say 'we got the wind knocked out' of us.
One hard hit to the chest or abdomen, for example, can cause the diaphragm to spasm or even paralyze for a moment. The lungs empty, and gasping for air is impossible for at least a couple seconds. It's scary and intensely uncomfortable.
Most of the time, the diaphragm will naturally relax and force an instant deep breath within a couple of seconds. If you have the wind knocked out of you, try to remain calm and when you take the deep breath, remain still, breathing deeply and steadily. There should be no long-term consequences.
However, if a person is hit in the chest and faints, this can mean the blow affected the heart and it could be an emergency. If a person falls and hits their head, that can suggest a concussion and the person should seek immediate medical attention.
Interestingly people can also become become acutely winded, meaning severe shortness of breath. The causes for this are usually more serious than phrenospasm. It can occur with anxiety or lung disease. This should be evaluated by a doctor.