As your Apple Watch cannot be unlocked via the normal process, then follow these steps to erase your locked Apple Watch and to re-pair it to your iPhone:
- Connect the Apple Watch to its charging cable, connected to power.
- Whilst keeping the Apple Watch on the charger and connected to power:
- 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 and confirm when prompted.
- Re-pair the Apple Watch using the Watch app on your iPhone, choosing to Restore from Backup if offered the option.