https://www.theverge.com/circuitbreaker/2018/10/30/18045762/apple-watchos-5-1-update-brick-pulled

Apple released watchOS 5.1 earlier today, but has temporarily removed it from availability after users reported that their watches were rendered unusable by attempting to install the update. 9to5Mac’s Guilherme Rambo experienced the issue, as well as several MacRumors readers.

Serious issues with the Apple Watch can be more complicated to resolve than with iPhones or other devices, because there’s no way to plug the device into a computer and reinstall the software yourself. The only recourse tends to be contacting Apple for service. The issue seems limited to the brand new Apple Watch Series 4, too, which is likely to compound frustration for affected users.