What happens if my Apple Watch dies during a workout?
Firstly, as it has died it will not track any more exercise.
Depending on what OS your Apple Watch is running, when you power it back up it will either sync the workout to your phone (phone must be nearby) or it will continue the workout.
We advise that you watch out for this when you put it back on the charger.
Back before OS4 you would lose all your workout data. Boy did that suck.