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, tap on Erase all content and settings and confirm when prompted.
- Re-pair your Apple Watch using the Watch app on your iPhone, choosing to Restore from Backup if offered the option.