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:
If your Apple Watch won't charge or it won't turn on - Apple Support
Note that 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