If your heart is not pumping adequately, it almost always due to heart muscle damage. The damage can be ischemia related (blocked vessel not providing oxygenated blood or enough to prevent the heart cells to die or become dormant), muscle damage can be due to a virus, alcohol, medication, drugs, etc.
The process of necrosis can be sudden (clot, etc.) and called a heart attack, cardiac arrest, MI or it can be gradual as is the situation with ischemia, etc. and referred to as silent heart attack.
I have a completely blocked LAD. My Dr said I had a silent heart attack so it was to late to do a bypass. I currently experience shortness of breath, chest tightness and exhaustion. I am told that I had Rheumatic Fever as a child but I cannot remember it. Also they said it could have happened from a virus. Because the heart attack was silent we don't know how long ago it happened but there is to much damage for a bypass. My other arteries are in very good shape so they don't think it was cholesterol.