It is a partial loss or loss of consciousness, temporarily. In other words, you have fainted.
 
A shortage of oxygen to the brain, leads to a "black out" episode, a loss of consciousness. Temporary impairment of the blood supply to the brain can be caused by heart conditions and by conditions that do not directly involve the heart.

Non Heart Conditions are: Drop in blood pressure due to changing body position to a more vertical position after lying or sitting; like standing up real fast; dehydration, a very fast drop in blood pressure due to blood pressure medication, a migraine, brain stroke, fainting from (ex:) blood drawing, coughing, swallowing, defecating, urinating, disease of the nerve to the legs in people who have diabetes or Parkinson’s disease.

Heart conditions that can cause temporary loss of consciousness: Wide spread disease of the heart muscle, tears in the aorta, abnormalities of the heart valves and high blood pressure.

No treatment is needed for non-heart conditions, while the heart condition patients may have their medicine reviewed.