During my swim, my Forerunner 945 suddenly turns off and not restart; watch power was 85%. I tried to press the upper left start button but it doesn't response, and not even charging now. What to do and what's wrong? Thanks.

After recording my swim for 30 mins, my Forerunner 945 suddenly turns off and not restart; watch power was 85%. I tried to press the upper left start button but it doesn't response, and not even charging now. What to do and what's wrong? Thanks.