iWatch Series 3 bricked (won't boot)

Hi,

after an automatic update, my iWatch Series 3 won't even boot. Simply shows the Apple logo until it runs out of battery.

I tried pressing both buttons simultaneously for several seconds to see if I can restart it but it just keeps hanging on the boot process.

The watch doesn't seem to be even paired to the iPhone anymore (I checked in the "Watch" App) so I can't restore it from my iPhone either.

Does anyone have a suggestion on how to recover it? Is there something else I can try before having to go to an authorized repair service?

Thank you in advance,

HJ