Hi
If you have already turned off Lost Mode by following the instructions here:
If your Apple Watch is lost or stolen - Apple Support
Then it may help to restart your Apple Watch (normally):
Restart your Apple Watch - Apple Support
Otherwise, if it remains locked and you cannot recall the password that was set for your Apple Watch, then follow these steps to erase it 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.
- Pair your watch using the Watch app on your iPhone, choosing to Restore from Backup if offered the option.
More information: