Hi
If your iPhone believes that your watch has been unpaired, but the process was not completed on your watch, you will need to reset your watch manually.
- On your watch, go to: Home screen (via a single press on the Digital Crown) > Settings (cog icon) > General > Reset > Erase all Content and Settings.
You should then be able to pair it via the standard process:
Set up your Apple Watch - Apple Support
If that does not work, try pairing manually instead:
- Check that Bluetooth is enabled on your iPhone: Settings > Bluetooth.
- Check that your iPhone is connected to Wi-Fi or to a cellular network: Settings > Wi-Fi.
- Ensure that both devices are charged.
- Place your watch on your wrist, with the band fastened and keep your iPhone nearby.
- Choose a language on your watch.
- Separately on each device, tap on Start Pairing (on your iPhone, do this within the Watch app).
- Tap Pair Apple Watch Manually on your iPhone.
- Tap the info icon "i" on your watch (it will be shown now). Your device's name should be displayed on the watch face.
- On your iPhone, tap the device name matching that shown on your watch face.
- A six-digit code will be displayed on your watch. Enter that code on your iPhone to pair the two devices.
- Then follow the steps as prompted to Set Up.