Start by ruling out software problems, there are a few simple things you can try from the comfort of home which will fix most software issues. In order of increasing time commitment...
- Put the watch on charge at least 30 minutes, 1 hour is better.
- Check your software versions, you should (today) have iOS 11.4 and WatchOS 4.3.1. Update if necessary.
- Forced reset the watch. Press and hold both buttons until the Apple logo appears and the watch begins to restart.
- Unpair the watch, then re-pair. This will force reloading all the watch software. You can restore from backup made during unpairing, or if that fails repeat and setup as a new watch.
Set up your Apple Watch - Apple Support
If these steps don’t fix the problem Contact Apple Support, see if they have any other ideas, and if all else fails arrange to get your watch examined at a Genius Bar appointment or Apple Service Center.