Myocardial infarction results in collapse, sometimes if help is available and the condition is recognized resuscitation can be performed; fibrillation results in weakness and often collapse and again, if recognized, measures such as defibrillation can be performed and normal rythmn restored.
Blood clots can travel from the lower extremities to the heart or lungs and result in collapse asymptomatically to the patient.
Sudden cessation of the heart without a blockage or a demonstrated arrythmnia is possible but quite rare among patients under normal medical care.
arrhythmia
My memory is telling me that a P.E.(pulmonary embolus) generally gives chest pain.