Unplug the affected speaker from power for a couple of minutes and reboot your router and phone.

I have the same issue with my ARC.  All of a sudden it’s not working.

Hi Unfortunately no joy  :-(

 Tim 

Have you tried a reset of that one problematic speaker only and adding it back to your Sonos system (Settings/System/Add Product) to see if that resolves the matter?

Hi Ken  thank you for replying but I don’t seem to have that option  of reset any speakers  let a lone the one not working 

Don’t attempt to Factory Reset the speakers that are working.

If you cannot Factory Reset the problem speaker, hardware failure is likely.. I suggest that you submit a diagnostic, log the confirmation number, and work with SONOS phone support.