How can someone be fine one night, then hours later die of heart failure?

It is possible: That's a sad scenario but not impossible. Some forms of heart disease strike abruptly. The most common by far is heart attack but there are others. Someone with few symptoms can rupture a plaque in an artery, occlude it, develop cardiogenic shock and die of heart failure in <24 hours although rare with modern treatment.

Arrhythmia: Patients with heart failure are prone to heart rhythm abnormalities (arrhythmias) which are fast, irregular, and don't pump any blood. If this occurs when the patient is alone, it results in sudden death. If emergency medical technicians are rapidly available, the heart rhythm can be shocked back to a regular rhythm with a small chance that the patient will fully recover.