Interrupted watchOS update
Just to show that this stuff happens to the ‘experts’ too, here’s an embarrassing story. I was updating my Nike+ Apple Watch to watchOS 4.0 this morning before work, having mistakenly believed it would take only half an hour or so. When it took longer, and it was time to go, and despite an onscreen warning specifically saying not to take the watch off its charger before the update was complete, I did exactly that.
Well, the Apple Watch promptly stopped working on my way to the train station, able to offer only an error screen with the URL of Apple’s support page. Restarting didn’t work, and nothing could be done from my connected iPhone either. In the end I had to contact Apple, who were very understanding and agreed to replace the device.
So if you’re getting the error screen and you just interrupted a watchOS update, there’s your answer. Go through the usual recovery steps by all means, but my humbling experience suggests that you may have just bricked your Apple Watch. Give Apple support a call.