Twice in a week I have started my watch on an activity and it has just stopped and not recorded anything. fenix 6 pro solar. How do I get back the distances that I lost?

If no distance has been logged then that is lost. You can manually add the distance if you want to log a distance. Log a 1 second activity, save and sync. Then edit that activity and set the correct date, time and distance and any other data that you might know.