That seems odd. I’d probably do a bunch of troubleshooting to see where the fault lies, be it hardware or software since there was no update yesterday, it’s hard to imagine it was software related, but worth checking anyway). 
 

In no particular mental order, I’d probably separate them as a stereo pair, and see if the speaker begins to work when grouped, rather than as a stereo pair. I’d also want to check the cables, both to make sure they’re seated properly (with no corrosion potentially ‘blocking’ the signal on either end of the cable). Finally, with them reinstituted as a stereo pair, and still exhibiting the issue, 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 Support staff, they have tools at their disposal that will allow them to give you advice specific to your network and Sonos system.