Apple Watch
1st generation Apple Watch Won’t charge or turn on after charging for over an hour. Tried turning on and resetting but still won’t turn on. Haven’t used it in about a year.
Apple Watch, watchOS 2
User profile for user:
Community User
Sounds like this could be your problem;
Lithium-ion batteries contain a protection circuit that shields the battery against abuse. This important safeguard also turns the battery off and makes it unusable if over-discharged. Slipping into sleep mode can happen when storing a Li-ion pack in a discharged state for any length of time as self-discharge would gradually deplete the remaining charge.All batteries are affected by self-discharge. Self-discharge is not a manufacturing defect but a battery characteristic.
Information from;
batteryuniversity.com