If an app isn’t working right on your Apple Watch, you can force it to quit by holding the side button until the sliders appear. Then let go of the side button and repress it until the app briefly reappears, then quits, and the Home screen appears.
If things still aren’t working correctly, try restarting or resetting Apple Watch and its paired iPhone.
Restart Apple Watch: To turn off Apple Watch, press and hold the side button until the sliders appear, then drag the Power Off slider to the right. To turn Apple Watch back on, hold down the side button until the Apple logo appears.
Restart the paired iPhone: To turn off iPhone, press and hold the Sleep/Wake button until the slider appears, then drag the slider to the right. To turn iPhone back on, hold down the Sleep/Wake button until the Apple logo appears.
If you can’t turn off Apple Watch or if the problem continues, you may need to force Apple Watch to restart. Do this only if you’re unable to restart your Apple Watch.
Force Apple Watch to restart: Hold down the side button and the Digital Crown at the same time for at least ten seconds, until the Apple logo appears.
To Reset Apple Watch settings: On Apple Watch, open the Settings app from the Home screen, go to General > Reset, then tap Erase All Content and Settings. After the reset finishes and Apple Watch restarts, you need to pair Apple Watch with iPhone again — open the Apple Watch app on iPhone, then follow the instructions shown on iPhone and Apple Watch.
FYI: Apple Watch content backs up automatically to your paired iPhone, and you can restore it from a stored backup. Apple Watch backups are included as part of your iPhone backups to iCloud or iTunes. When paired with an iPhone, Apple Watch content is backed up continuously to iPhone. If you unpair the devices, a backup is performed first. If you repair Apple Watch or get a new one, you can choose Restore from Backup and select a stored backup on your iPhone.