A heart attack is an event that destroys
heart muscle due to lack of blood/oxygen supply to
heart cells. The lack of oxygen
causes cell necrosis, but sometimes when treated appropriately within 72 hours the cells may be revived (heart cells were dormant).
A heart attack can be without symptoms, or have symptoms chest pain, etc. About 5 years ago my first symptoms of having had a "silent" heart attack were symptoms of congested heart failure.