Hi,
If it works on the charger but not when on the battery only, then most likely the battery has failed and needs to be replaced.
here’s a link to the ifixit [guide|86653] guide, which shows how to replace the battery.
ifixit sell a battery for the watch. Here’s a link [product|IF384-000]
There are also other suppliers that may suit you better. Just search online for ''LSSP321830'' to get results.