The heart of SmartThings is their cloud service that powers the iOS and Android apps, device control, adding devices, device integrations, handlers, voice assistant integration and more.  The SmartThings cloud service has been incredibly robust and reliable.  If you're viewing this page however it's possible that you're having a problem with your hub showing offline or you are experiencing connectivity problems.

The first step is to check the SmartThings Status Page found here:

It will show you if the SmartThings cloud service is working normally, experiencing an outage or downtime, and if they are on top of any issue that may be occurring.  They also post information about certain firmware rollouts here.

You can also subscribe to updates from this status page so you'll be proactively notified of a problem and resolution.