The first thing I’d try is unplugging them both from power, then rebooting the router. Wait for the router to come back up before plugging them back in. It sounds like they’ve lost sync, and forcing them to reload the OS may help. 

If it continues, though, I’d 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 network and Sonos system.

Thanks, I’ll give it a go again and see what happens. Failing that, I’ll get in touch with support. I’ll keep everyone updated in case anyone else is experiencing the same problem. Have a nice weekend