Today I went for a run, the activity was saved to my Forerunner 225. Went I got back I synced it using the Garmin Express app, which said the sync was successful. When I opened the Garmin Connect webpage the new run was not there. After ejecting my watch I saw that not only was the newest activity missing, but my entire history was gone. The history still shows up on the Garmin Connect webpage (minus today's run), but it's gone from the watch. The GARMIN/ACTIVITY folder is empty. I am able to start a new activity and save it, the activity can be seen from the watch and in the GARMIN/ACTIVITY folder. But my entire history going back several years is gone.
What could be the reason for this and how can I avoid it in the future?