Unfortunately, it is not possible to reset a stolen Apple Watch. If your device has been stolen, you can use the Find My app on your iPhone to mark the device as lost. This will help prevent anyone else from being able to access your data or use Apple Pay. You can also contact local law enforcement to report the theft and take other appropriate steps. If you have AppleCare+ with Theft and Loss, you can replace the device and restore your data. However, it cannot be reset. To help protect yourself in the future, enable a passcode on your Apple Watch and keep it up to date with the latest software updates. Additionally, consider setting up two-factor authentication on your Apple ID to help protect your account and data.