Follow the steps 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:
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, if you have access to one).
Note that, depending on how long the Apple Watch has been stored, the battery may have fallen into a deep discharge state and now be incapable of holding a charge.
Apple advises that, when storing Apple Watch long term, you should store it with a 50% charge and then recharge it to 50% every six months. See under the heading "Store it half-charged when you store it long term":
Batteries - Maximizing Performance - Apple