Sounds like the speaker(s) are having trouble communicating with your controller device, but not the router, something that seems rarer than average. It may be useful to know which version of the Sonos OS you’re using, as well as your router type and manufacturer, as well as what device you’re running the controller on.
Some rather generic advice, which certainly won’t hurt, try removing power from your Sonos devices, and turn off (and I mean power down) your controller device. While all of these are ‘off’, reboot your router. Wait two or three minutes for the router to come back up (it’s forever, I know), then you can power on both your controller device, and the Sonos devices. All of them will get fresh IPs, in addition to a reload of all the various software installs. Wait another couple of minutes to allow everything to boot up and reconnect before testing.
Also, it might be worth reading the wifi interference FAQ.
Finally, I would recommend that you 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.
Get it to happen, submit a diagnostic and call Sonos Support to have them look into what is happening.