Hi
Follow these steps to erase your locked Apple Watch and re-pair it with your iPhone:
- Connect your watch to its charging cable, connected to power.
- Whilst keeping your Apple Watch on the charger:
- Press and hold the side button until you see the Power Off slider.
- Press firmly on Power Off (do not try to slide it - just press down more firmly than a normal tap), release your finger and then tap on Erase all content and settings.
- Re-pair your watch using the Watch app on your iPhone, choosing to Restore from Backup if offered the option.
More information:
Set up your Apple Watch - Apple Support
If you forgot your Apple Watch passcode - Apple Support
Apple's instructions for turning off your Apple Watch and for restarting it are detailed here. Note that it should not be force restarted unless it cannot be restarted normally: