If your watch doesn't start, you will have to reset it. There are 2 ways to reset:

1. Resetting the watch: Accessible from the Settings> About > Reset menu, this resets all your custom configurations and settings and deletes all sessions in the memory
   
   
2. Rebooting the watch: If your watch freezes on a screen or no longer responds, you can start it again by pressing the ON/OFF button (top right on the watch) for 20 seconds