Apple released watchOS 5.1 last week alongside of iOS 12.1 and tvOS 12.1, and it only lasted a few hours before it was pulled from servers. As it turns out, it was bricking some Apple Watch Series 4 units, so the boot wouldn't get past the Apple logo.
Today, the firm released watchOS 5.1.1, and the main feature is exactly what you might have guessed: it doesn't brick your Apple Watch. It has the same list of fixes as before:
Apple Watch Series 4 automatically contacts emergency services if you are immobile for about a minute after detecting a hard fall. The watch will now also play a message that informs the responder that Apple Watch has detected a fall and shares your location coordinates when possible.
Fixes an issue that could cause an incomplete installation of the Walkie-Talkie app for some users
Resolves an issue that prevented some users from being able to send or receive invitations on Walkie-Talkie
Addresses an issue where some previously earned Activity awards were not showing in the Awards tab of the Activity app for some users
When an Apple Watch won't boot, that's particularly problematic because there's no way to recover it without a visit to the Apple Store. There's no Lightning or USB Type-C port on it that will allow you to plug it into a PC and restore a factory image through iTunes. Apple has to do it for you. This is also why installing watchOS betas comes with a stronger warnings than iOS, macOS, or tvOS betas.
The update still doesn't have the ECG functionality that was promised for the Apple Watch Series 4. That should still be coming later on this year, so expect to see watchOS 5.2 in beta soon enough.