Hi
Try force restarting your watch: remove your watch from the charger and press and hold both the side button and Digital Crown for around 10 seconds, until you see the logo.
If that does not help, next try charging your watch for around 2 hours, making sure that you follow the charging instructions here:
Check your battery and charge your Apple Watch - Apple Support
After that time, if it has not already restarted automatically, remove it from the charger and:
- Try restarting it (by pressing and holding the side button until you see the Apple logo).
- If necessary, next try force restarting it again.
If the issue still persists, you will need to contact Apple Support, make a Genius Bar reservation or visit an Apple Authorised Service Provider to arrange for your watch to be serviced: