Sometimes after a Sonos update (such as the current one that’s available now) you may see in System Settings that a component is not connected.  The warning may appear even though there was no “Update Failed” screen at the end. Unless you can readily identify the room containing the component discovery may be a PITA 

Sonos suggests following the steps in this link. Those steps can appear a bit daunting at times.  Especially, if you have several Sonos components of the same type.

Here are some steps you can try that are less time consuming and may yield quicker results:

• Wait a few minutes to see if the component(s) reappears and the warning goes away
• Close the Sonos app and Reopen It
• Stop the Sonos app and Restart it _iPhone users click here  / Android users click here
• Start a music stream to the room via Spotify, Apple Music, SirusXM or other service to awaken the component
• If none of the above resolve the issue then follow the steps suggested by Sonos in the link above *

 * Never Factory Reset your Sonos components; unless directed to do so by Sonos Tech Support