Have a look at these steps:
...
If you find your missing Apple Watch, tap Unlock on your watch, then enter your passcode. Lost Mode will automatically turn off, and you can start using your watch again as normal. You can also turn off Lost Mode from your paired iPhone or iCloud.com.
On the web
- Sign in to iCloud.com with your Apple Account.
- From the Quick Access Menu in the top right, select Find My.
- In the All Devices menu, select your Apple Watch.
- Select Lost Mode > Stop Lost Mode, then select Stop Lost Mode again to confirm.
On your device
- Open Find My, tap Devices, then select your Apple Watch.
- Tap Lost Mode.
- Tap Turn Off Lost Mode, then tap Turn Off to confirm.
...
copied from: If your Apple Watch is lost or stolen - Apple Support