Your Bridge sticks out as a possible source of connection trouble, though I must say that it probably would not close the app. The Sonos Bridge’s power module is prone to failing. So either get a new power module, cable connect one of your devices to your router, or connect your devices via WiFi. Also take a look at https://support.sonos.com/en/article/remove-a-bridge-from-your-sonos-s1-system

Your app is just a remote control - it tells the speakers which music they should seek from what source, and shows what they are playing. So if the app itself is crashing, it ought to be an issue local to your phone/device rather than your entire system (as ​@106rallye initially alludes to). 

I’d first try restarting your phone. Also, there has been an iOS software update this week - have you installed that? (Edit: you said your OS are all up to date, so I suspect you did install that.)

Yes my apps are all updated and I cannot fathom why the app opens and shuts immediately. Tried the usual restarts and uninstalls / reinstall and nothing has helped! Grim.

One thing to ask - when did you last run a System Update on your speakers? If not for a while, download the app to another device, including maybe Windows or Mac (from the Sonos website), and run the System Update in Settings to ‘Check for Updates’. Update the speakers if directed.

If everything is confirmed as up-to-date then try the following, in this order: 

• Force quit the app.
• Turn off the phone.
• Turn off the router at the wall.
• Turn off your speakers at the wall. 
• Wait a full five minutes. 
• Turn on your router and let it fully boot up.
• Turn on your speakers, one at a time letting each boot up to white light before moving to the next one.
• Turn on your phone.
• Re-open the Sonos app. Does it stay open this time??

Thanks for this checklist, really useful. I will give it a go. According to my app, that’s up to date, so assumption is that the speakers / system are also up to date? Will go through your suggestions and see where I end up and report back! Thank you

App updates and System updates are different things. One just updates the app on your phone, whereas the latter ensures your speaker firmware is all up-to-date and has the latest functionality. Give it a try and see how it goes. (If the speakers DO need an update, do that, and then force quit and re-open the app to see if that might have cured it. Failing that, run through the checklist.)

I downloaded the Sonos controller on my Mac and checked for updates for the system and it was all up to date. Tried some music and all worked ok from the Mac controller- then went back into my iPhone Sonos app and it worked! Coincidence? Very bizarre! Thanks for your help though. Much appreciated 

Glad to hear it! Maybe it just needed a jolt from another controller to clear a bug through. 

But should anything go strange in future, maybe keep that list of actions to hand, just in case...

List safely copied! Thanks again