Yes you can die instantly from a heart attack if the heart attack is severe enough.
Some people who have a heart attack fall down instantly and die instantly from the heart attack and sometimes they fall down and then take a few minutes or so to die from the heart attack especially if they have no medical attention available.
If you think you're having a heart attack call 911 and get an ambulance on the way just to be safe because you might have a small heart attack and feel the chest pain and other symptoms of the heart attack.
Then the heart attack symptoms and feelings may go away and then you might also have another heart attack and die.
So always get checked out and get emergency medical attention for the heart attack.
Most people who experience heart attacks do eventually recover as long as they get the medical attention when needed.