Apple Watch doesn’t charge

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. For more info 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

If the battery drains too fast after an update, see all the replies to:

Update 6.2.1 Battery Draining Too Quickly… - Apple Community

Also see:

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

If you still need help, contact Apple Support.

If you stored your Watch for a long period of time at less than 50% charge, the battery is likely damaged beyond further use.

Apple doesn't usually repair Watch batteries, they replace the watch with the same remanufactured model. Since you stored the watch improperly, even if the watch is still under warranty, the battery damage isn't likely covered by the warrnaty.

You will likely need to buy a remanufactured replacement at an out-of-warranty price. A bit pricey, depending on the model, but less than the cost of a new Watch.

Apple Watch Service and Repair

https://support.apple.com/watch/repair/service

M1 Mac mini, macOS 11.2; Watch, series 5 GPS/cellular, watchOS 7.3; iPad Pro 2nd Gen, iPadOS 14.4; iPad Air 2, iPadOS 14.4; iPhone 12 Pro, iOS 14.4; TV 4th Gen, tvOS 14.;  HomePods, audioOS 14.4 and Airport Express w/ Airplay 2