Sorry, are you asking for help?  If so, can you explain your issue?

Where shall I begin! 
Sonos disappearing completely. All speakers suddenly silent. Multiple times everyday

Try to open Sonos app. Suddenly no access.

Click on app.
You get a Black Screen

when you can open the app  it suddenly decides you haven’t got all the speakers you know you have

 The app can’t find the speakers in your system

Or can find only 1 of them but not the others 

Its utter rubbish 

Do you have a mesh network? Are any of your devices wired via an ethernet cable? If no wired devices is this possible to do as a test? And if you do have a mesh WiFi network can you wire the device to the base unit.

You and I have the same app, and different network setups.  My Sonos system and app work fine, yours don’t.  Where shall we look for the problem?

Please describe your network setup.  Make and model of router?  Any mesh network, access points, extenders etc?

Perhaps I didn’t make it clear.
My point is that it all worked fine  until the upgrade. 
 I do have 1 linked with an Ethernet cable. 
I have put up with all the Nighles since  the upgrade. 
Such as ‘can’t find your products’

Suddenly switching off after playing for several hrs. etc  

Today was just the last straw. It’s the worst App ever. 

I’ll have one last try.  The app is just a remote control for the Sonos system.  It is very unlikely to be the cause of the type of problem you are having.  I have found over many years that when there hasn’t been a  recent update, people come on here and say “my system stopped working”.  If there has been an update they say “the update broke my system”.

But if you don’t want to consider any other possibilities, I wish you well in your Sonos-free future.

Thanks for your responses. My Sonos system is working fine without using the App. I appreciate the App is just a remote. It’s a really awful remote.