A walk across a room leaves him exhausted. Climbing the two flights of steps to his apartment might take half an hour. His legs swell. And he has to sleep sitting upright in a chair. He feels like he is dying.
People often feel like that when their hearts are failing.
But without warning, his heart stopped doing its job. Heart failure means that the organ can't pump enough blood to meet the body's needs. Usually it's not contracting well. Sometimes, it can't relax sufficiently. There are often many symptoms, but the most common are shortness of breath and fatigue when exercising.