If you haven't already done this, got to Settings > Apps > Sonos, and make sure Local Network is enabled.

Hi ​@Pools-3015 
The Local Network is already enabled.  I am on the correct wifi and can see all of mu devices .

I cannot control any of the via the APP.   Will update the symptoms too.

Any chance you have a mesh network? It could be the iPhone is connecting to a different subnet than the windows machine and the speakers. 

Indeed I do have mesh network.   .   All the devices are showing on the same SSID and also the 5Ghz section.   
Would you believe it has started working again today! 

I suspect, without a lot of proof, that your mesh is breaking up your network into different subnets, which Sonos can’t communicate across. Which is a level different from the SSID in the network stack ;)

I can believe it started working, though, it may have changed, networks are designed to constantly adjust, where they can, to variables. 

I don’t use a mesh myself, but you might check to see if there is a setting to maintain a single subnet. Or if it reoccurs, try disconnecting any extender ’pucks’, leaving only the main router temporarily, and see if it resolves the issue. This will confirm whether my guess is correct or not (mostly…there could be other things going on).

If you prefer, the next time it occurs, I would recommend that you submit a system diagnostic within 10 minutes of experiencing this problem, and call Sonos Support to discuss it. But 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. And unfortunately, both you and I don’t have access to see that data.

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.

For what it’s worth, and depending on which Sonos devices you have, I don’t think reverting back to S1 is a great option. It hasn’t received any of the service updates that have occurred with S2, and certainly won’t run many of the newer Sonos speakers.