That suggests some unspecified network issue, with your remote (the controller app) not maintaining a connection with the data on your system (speakers/devices). 

I would start with a simple network refresh. Power off all Sonos devices. Reboot your router. Wait two minutes, then power back on your Sonos devices. This will resolve some, but not all potential issues, there just isn’t enough detail to be certain why what you’re experiencing is happening. 

If that doesn’t help, I would recommend that you 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.

Bruce, thanks for the detailed response. I'll do as you recommend.