What happens when there is a blood blockage in the coronary arteries?

The arteries that bring blood to the heart muscle (coronary arteries) can become clogged by plaque (a buildup of fat, cholesterol and other substances). This can slow or stop blood flow through the heart's blood vessels, leading to chest pain or a heart attack. Increasing blood flow to the heart muscle can relieve chest pain and reduce the risk of heart attack.
If you are suffering chest pain or other symptoms that may indicate a cardiovascular problem, undergo an angiogram (cardiac catheterization) to see if your coronary arteries are blocked by plaque. A blockage can cause a decrease in the supply of blood and oxygen to the heart and over time can lead to debilitating chest pain or a heart attack. Plaque material consists of fat, cholesterol, calcium and a variety of other cellular components like smooth muscle cells and blood cells. The deposition of this material occurs almost throughout life, starting even during childhood in many people. As this deposition progresses, the lumen gradually narrows and the blood flow is limited. The plaque material can also be somewhat brittle and fragile. Sometimes, the plaque cracks or fractures causing an acute obstruction with a sudden build-up of platelets which completely closes the lumen and flow ceases altogether. Most commonly, a heart attack then ensues.

References:

http://http//www.slrctsurgery.com/cabs.html

http://http//www.daviddarling.info/encyclopedia/C/coronary_artery_bypass_grafting.html

Glossary:

Cardiovascular disease or cardiovascular diseases refers to the class of diseases that involve the heart or blood vessels (arteries and veins).
Cardiac catheterization (heart cath) is the insertion of a catheter into a chamber or vessel of the heart. This is done for both investigational and interventional purposes.