Heart Failure does not mean the heart has stopped working. It means the heart is not working at its fullest capacity. When the heart fails, heart output is diminished and other organs don't receive the oxygen and nutrients they need to function normally. There are two types of Heart Failure: Systolic Dysfunction and Diastolic Dysfunction.
The most common cause is a Heart Attack. Other causes are: Coronary Heart Disease, High Blood Pressure , Heart Valve Disease, Thyroid Disease, Kidney Disease, Diabetes or heart defects present at birth.