Why my Apple Watch won't charge or it won't turn on

Blank screen, just red charge symbol and time when button pressed, checked charging components and ok. Watch on charge device changes red charge symbol to green but still no watch operation. On charge all day (10 hours) Tried forced reset during day but no change.

After 12 hours on charge finally got normal watch charge display. 1 hour later reached full charge and normal operation.

What would have caused this malfunction?

Thanks for your advice, much appreciated. I think the problem arose because I allowed my watch battery to discharge past minimum disabling it. Glad it’s back as was just about to send it for repair. Anyway it’s fine now.