https://www.theregister.co.uk/2018/11/01/apple_watch_4_borked_update/

Apple has withdrawn a faulty update that has left Apple Watch 4 owners with bricked bling.

The watchOS update (5.1) to the new kit has resulted in an endless reboot cycle for some, rendering the device useless. Watch owners at MacRumours and Reddit flagged the issue as major.