Yes people can suddenly die and just drop dead without any other known health issues.
It's possible to suddenly have a heart attack and go into cardiac arrest even if you've been healthy all the time.
You could have a healthy heart and then suddenly you go into cardiac arrest and then can drop dead or at least go unconscious and never wake back up if you don't get medical treatment.
You can also die suddenly in your sleep due to cardiac arrest and some people do go to sleep and then have a heart attack in their sleep and die.
I was working with a guy in a field before and he suddenly clenched his chest and dropped to the ground.
He was never able to be revived and died suddenly.
The guy was in great health, exercised and then suddenly died and dropped dead out in the field.