Heart attack occurs when blood flow to a section of heart muscle is blocked. If the flow of blood isnt restored quickly, that section of heart muscle becomes damaged due to lack of oxygen and begins to die. This happens when a clot in the coronary artery(artery that supplies blood and oxygen to your heart) blocks the supply of blood and oxygen to the heart, leading to an irregular heartbeat and severe decrease in the heart's pumping function and may cause death.
Attribution: Straight Talk Foundation