I’d start by resetting your network. Unplug all Sonos devices (PLAY isn’t really a good description, there are PLAY:1s, PLAY:3s and two different generations of PLAY:5s, and you could be using S1 or S2), then reboot your router. Wait 2 minutes, then plug in your Sonos devices. If there is still one missing, connect it to your router with an Ethernet cable, and make sure the WiFi / radio is turned on. It may be worth flipping that virtual switch back and forth, just to make sure the radio is on. If it disappears after you remove the Ethernet cable, there may be a WiFi system issue, in which case I would connect it so it shows up, then submit a system diagnostic within 10 minutes of experiencing this problem, and call Sonos Support to discuss it. Don’t post the resulting diagnostic number here, they get sensitive about GDPR.

There may be information included in the diagnostic that will help Sonos pinpoint the issue and help you find a solution.

When you speak directly to the Support staff, they have tools at their disposal that will allow them to give you advice specific to your network and Sonos system.