Apple Watch has no battery and shows snake design. It is not charging
Apple Watch is not charging and shows only snake sign
Apple Watch Series 3, watchOS 8
Check out all your electrics as described in this article
If your Apple Watch won't charge or it won't turn on - Apple Support
After that you may need to contact Apple for hardware intervention, hopefully just a new battery
Genius Bar Reservation and Apple Support Options - Apple
and/or