If you store a device when its battery is fully discharged, the battery could fall into a deep discharge state, which renders it incapable of holding a charge. To understand Apple Watch performance and its relation to your battery, see:
Apple Watch - Battery - Apple
Apple Watch Battery and Performance - Apple Support
“Battery life” is the amount of time your device runs before it needs to be recharged. “Battery lifespan” is the amount of time your battery lasts until it needs to be replaced. Maximize both and you’ll get the most out of your Apple devices, no matter which ones you own. See the "Tips for Apple Watch" section in:
Batteries - Maximizing Performance - Apple
For repair & service:
watch Repair - Official Apple Support
watch Service Pricing - Apple Support
You can try:
If your Apple Watch won't charge or it won't turn on - Apple Support
If you still need help, contact Apple Support.