Try restarting (or, if that doesn't help, force restarting) your watch:
- Restart Apple Watch: Press and hold the side button until you see the Power Off slider; Drag the slider to turn off completely; Afterwards, press and hold the side button until you see the Apple logo.
- Force restart: press and hold both the side button and Digital Crown for at least 10 seconds, until you see the Apple logo
Check also that:
- All plastic wrapping has been removed from both ends and both sides of the charging cable (check very closely, especially on the concave / curved white section that attaches to the back of your watch).
- There is no debris on the back of your Apple Watch, the adapter or the magnet.
- The charging head is directly aligned with the back of the watch and is touching it fully (some stands / charging stations might hinder this).
- You are using only the genuine Apple Magnetic Charging Cable.
- When plugging into mains power:
- You are using a genuine Apple USB Power Adapter.
- Besides the adapter that is supplied with the watch, the cable can also be used with a 5W (iPhone) or 12W (iPad) Apple USB Power Adapter - test also with these if you have them.
- When charging via USB, ensure that:
- You use a USB 2.0 or 3.0 port.
- The computer remains on and awake.
- The cable is plugged directly into the computer (not into, for example, an external keyboard).
- If your Apple Watch is out of battery power, you may need to wait for a few minutes before the green lightning bolt appears (confirming that charging has started).
Check battery power and charge your Apple Watch - Apple Support
If you have followed these steps (including trying again to restart or force restart your watch after any further charging) and are still having problems:
Contact Apple Support with a view to having your watch and/or power accessories checked and, if necessary, replaced under warranty:
https://www.apple.com/support/watch/contact/
Or make a Genius Bar reservation at your nearest Apple Retail Store:
US: http://www.apple.com/retail/geniusbar/
UK: http://www.apple.com/uk/retail/geniusbar/
For other countries, visit: http://www.apple.com/choose-your-country/