It's unclear what caused the problem because no update went out today, but judging by reports, running through setup again is enough to get most people back up and running. Some owners with mesh networks are reporting that only one of their devices ever went offline, while there are others who are still disconnected even after a hard reset. Of course, for those with any kind of custom settings that could cause quite a headache. We've contacted Google about the issue and will update this post when we receive a response.

Update: The Wifi Care Team has posted again on the forum, and says that it will require a hard reset for affected users to get back online. Unfortunately, it appears these devices don't have the capability to backup their settings, so owners will have to redo any customizations.

On a special support page for the incident, it has specific instructions for varying situations, and claims the problem occurred because of an issue with the Google Accounts engine.

Google: