Follow the instructions here, including contacting Apple Support (mail-in service may be available, if required), making a Genius Bar reservation or visiting an Apple Authorised Service Provider if necessary:
- Check your battery and charge your Apple Watch - Apple Support
- If your Apple Watch won't charge or it won't turn on - Apple Support
Be sure to use only an Apple or MFi-certified USB power adapter or third-party accessory in conjunction with the Magnetic Charging Cable or Magnetic Charging Dock when charging your Apple Watch. If you are already doing so, try using another such cable and/or power adapter (including trying an Apple iPad USB Power Adapter first, if you have access to one).