There’s simply not enough detail in your post to be sure about it, so I’ll offer a couple of suggestions.

First, wire one Sonos device (not a surround speaker or a Sub) to your router with an Ethernet cable, and wait a couple of minutes before opening the controller to see if things show up. If you already have a BRIDGE wired, wire a speaker instead of the BRIDGE.

Second, try a refresh of the Sonos network connections, by unplugging all Sonos devices from power, then rebooting your router. If the Sonos devices are not unplugged while you reboot the router, you’re not getting the full benefit of the process. Only after the router comes back up should you plug the Sonos devices back in to the power.

Report back, with more details about your set up, and perhaps the community can hone in on what in particular is causing this unusual behavior in your system. 

Let’s see, I’m not sure what more info you need that would be helpful?  We have tried ALL of the above, numerous times.  We have tried wiring individual speakers to the router with an Ethernet cable - nothing.  We have unplugged all Sonos products, rebooted the router, waited, plugged everything back in,  You name it.  Yesterday, OCCASSIONALLY a FEW of the speakers would come back and show on the app, then they would disappear again.  We have spent HOURS on the phone with both our internet service provider (Spectrum) and Sonos.  No one knows what the problem is.  Once we were told it was a firmware update from Spectrum- wrong - we’ve had no update. We checked.  At this point, I’m ready to sell $3,000 or so worth of products - anyone interested?  LOL

Our internet provider is Spectrum and our Router is an Askey/SAC2V1K.  We have a Boost, a Sonos Connect Gen2, multiple Play1s, Sonos1, Sonos 1SL, Play3, Play5s, Sonos Beam, and 2 Moves.  We’ve never had issues before.  This happened literally overnight.  We have an entire Smart Home, and Sonos is the only thing not working.  As a matter of fact, we can still play our Sonos speakers - kind of - if we tell ALEXA to play them! 

As I interpret the information, all Sonos devices were not unplugged while you were rebooting your router. But that continues to be a guess, based on what you’ve supplied in terms of details. And you’ve explicitly said that it’s not the case. But the simple fact that some devices show up sometimes certainly points to some issue with the speakers not being able to connect through your router to the controller, on whatever device you have it on. That tends to point to either duplicate IP addresses, or some sort of wifi interference . Since not all interference can be attributed to being inside your network, it could be an outside change that you’re not familiar with. For me, it happened to be new neighbors that were stomping in my SonosNet channel. In a prior home, it was a microwave leaking RF while it was on.

There are other options, since what we are talking about here it network connectivity, but duplicate IPaddresses remains my initial guess. Or, you could shoot that theory down by telling me you have assigned reserved IP addresses for all your Sonos devices in your router’s DHCP table.

But, I still don’t know which version number of the Sonos S1 controller you’re using is, or what device it’s on, what OS is running on the device that the controller is on, or what version of the software is running on each speaker. 
Another thing to consider is background updating of things like routers, firewalls (or VPNs) that tend not to announce that they’ve changed. A change in a VPN, for instance, could block the ability of your controller to see the speakers, while still allowing Amazon’s Echo devices to play to them without issue. 

Unfortunately, community members don’t have access to diagnostics, so all of this is based solely on what you’ve said, and my understanding of the way Sonos interacts with networking. 

Thanks, Bruce!  Unfortunately, most of this is far beyond my limited networking skills!  I can answer a few of your questions:

1.  We definitely unplugged every Sonos speaker, Boost, etc.  before rebooting the router again.  Then waited several minutes, then tried plugging in one of our Play1 Speakers via Ethernet.  Again, nothing.  
2. We continue to get intermittent access to one - four of our products, but it has never been close to all of them since Monday.  
3. When no speakers/products are visible in the app, we can’t even access our account or settings, so it becomes a completely “dead” app.
4. The S1 controller is 11.2.3 version on two iPhones:  an 11 and an 8, both running iOS 13.7 at the moment
5. The same thing is happening on my network, in that I periodically see my Sonos devices listed (right now there is one listed), but then they disappear as well. 
6. We don’t us a VPN (used to, but had so many problems we quit!)
7. When it comes to duplicate IP addresses, I’m stumped.  I basically understand the concept, but I have no clue how to track that down or fix it…
8. I also tried turning off the firewall and reloading the app, but again, nothing.  
9. I understand your comments regarding interference, to a point, but again, don’t really have a clue how I would track that down.  I do know that it’s not something as simple as a microwave, for example.  I assume this would simply cause intermittent outages of a fairly short duration?  We had no issues at all until Monday mid-day.  One minute everything was fine, and the next - “”Unable to connect to Sonos.”  
10. We plan to go back to our isp AGAIN.  So far, we can’t get past a wall of “technicians” who are reading from a playbook and have no idea what Sonos even is LOL.

I appreciate your thoughtful responses!

Christy 

Hi Christy, I have the same router and the same thing has happened to me. I have called both spectrum and sonos and no one has any answers for me. Its so frustrating. I will keep working and let you know if I am able to fix anything.

Update - get this - we have a TP Link AC-750 Wi-Fi Range Extender/Powerline.  My husband tried plugging it into the router and creating a “new” system - even with a new account, etc.  He connected a Boost and one Play5 and they are now working correctly.  Did this basically so he could go back to Spectrum and tell them the problem is the ROUTER, not Sonos or the App.  This won’t work for our entire system, of course, as it’s not powerful enough to do our 12 products that extend over 2 floors and indoor/outdoor.  But it sure proves its a router issue of some sort! 

Thanks for the update @CCS014 Are you going to still go back to spectrum or will you wait, or will you get your own router? I appreciate your posts because I was losing my mind this week and thought I was alone!

@Bevansburg - I know the feeling!  We are back on!!!!  I gather there was an update to the Spectrum router (that no one will really admit) - or some other Spectrum update.  It requires a new Firmware update to correct the new issues.   The problem we were having before is that no one there appeared to even understand what Sonos was, etc. - so they couldn’t figure out the problem in their list of “fixes.”  However, just after I wrote last we discovered that my printer was also offline.  Once we told them the PRINTER was wasn’t functioning, they knew exactly what to do.  I suggest you call Spectrum again and tell them the Sonos AND your printer quit working LOL.  Good luck to you!!!

And thank you also, Bruce.  You pointed us in the right direction and helped get us there.  Catastrophe avoided :-) 

I’m on with “support” now and all they want to do is factory reset the router - what did your support person “do.” my person is saying they can’t control firmware updates :eye roll:

This is Christy’s husband- I told them that our Wi-Fi printer was not being recognized by the system. From what the person told me they did a firmware update. He also stated that there of been many calls about this issue. I told her upfront if you couldn’t fix it I would want to speak with a supervisor. I suggest you do the same