A heart attack occurs when there is a block of flow of blood through the coronary artery which then cannot get enough oxygen. The coronary arteries becomes blocked from the buildup of various substances, including cholesterol. Heart attacks can also occur because of a spasm of a coronary artery, which shuts down the flow of blood to the heart muscle.

The condition called atherosclerosis is the built up of waxy substances in the arteries and it takes place over many years. When an area of this plaque ruptures and goes inside the artery, it causes a blood clot which ends blocking the blood flow through the coronary artery.

From this Mayo Clinic site I took the following image where you can see why the heart attack occurs heart attack