Please try this -
On your Apple Watch, press and hold the side button (not the crown) until you see Power Off or the small icon at the top, right of the screen. Follow the prompts to power your watch off. Wait several minutes then press and hod the side button again until the Apple logo appears.
Hopefully this restart will fix the problem. If it does not, please come back here and let us know. Also, if my instructions were too confusing, then follow this link to the Apple Support answer.
Restart your Apple Watch - Apple Support