Doesn’t make a lot of sense, honestly. The HDMI ARC connection is completely separate from the WiFi connection. One should not cause a failure in the other. I wonder if it’s coincidence on the timing, and your Beam fell off of WiFi, with no actual connection to the use of the TV. 

Id recommend a couple of things. Start with an refresh of your network, by unplugging all Sonos devices from power, then rebooting your router. Once the router comes back up, plug back in the Sonos devices, and see if, after a minute or two for them all to boot up, if they all show up again in your controller. 

If they don’t, wire an Ethernet cable (temporarily) between your router and the Beam, and check to see if the Beam shows up in the controller. If it does, submit a system diagnostic, and call Sonos Support to discuss it, or post the diagnostic number here for a Community Moderator to pick up.

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, there are more options available beyond just the diagnostic analysis.