I’d try a few things. First, unplug the speaker from power, and reboot your router. Wait a couple of minutes before plugging back in your speaker, and give it a couple of minutes before checking for it in the controller. Next, if that doesn’t work, I’d try connecting it to my router temporarily with an Ethernet cable, if possible. It’s ok to move it. Next, I’d be reading through the wifi interference FAQ, to see if there may be some outside influence on your WiFi signal.
Last, but certainly not least, I’d recommend that you submit a system diagnostic within 10 minutes of experiencing this problem, and call Sonos Support to discuss it.
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 phone folks, they have tools at their disposal that will allow them to give you advice specific to your Sonos system and network.