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

Sudden death: Heart attacks are caused from a clot in the coronary artery. 10-25% of heart attack patients can develop ventricular fibrillation within the first 8 hours that causes the sudden death if not detected and shocked immediately.

Yes: A great number of heart attacks can be sudden and fatal. The incidence of sudden death is higher with age, with organic heart disease and ejection fraction <=35%, critical left main coronary artery stenosis, critical aortic stenosis, previous unrevascularized myocardial infarction territory, acute aortic dissection specially in patients with marfan's syndrome, pulmonary hypertension.