Force quit the app. 

Turn off the router and the speaker(s).

Leave it five mins, then turn the router back on and let it fully reboot.

Then turn the speaker back on and let it go to white light. Do the same for any other speakers.

Re-open the app. Things should work again now.

(also check you are on the latest app version - not sure if the S1 app gets updates; and also run any available speaker updates from the app settings.)

What Rhonny said above - plus do NOT factory reset anything.

Oh and what device is the app running on?

If you have problems finding the Force Quit you can reboot the Controller's device instead.

Thanks in my haste I forgot to say what products I have.

Bedroom  -  One downgraded to work on S1 system

Living room - Playbar

Office - Bridge

Kitchen - Connect Amp

Use the Sonos S1 app on Apple iPhone.

I have had this problem before. (think its when I do App updates on phone?!). got so frustrated pushed every button going (including the ones in my head LOL) obviously something worked.  

Checked System under settings, and the connect-amp is missing.  Do the find the product, flashing orange light etc. then either failed retry or says “set up new network go to app and select the Sonos network, but can't get out of the app, close it down and reopen it, connect amp still not showing go to network and there is Channel 1 (with a tick) channel 6 and channel 11.  Can now see the connect amp but when I try and play even Sonos radio it says unable to connect.  it also shows my music library select a song and get “an error occurred while adding tracks to the queue” 

Going round in circles, is it the connect amp, my network or a problem with the app?

One last thing, just started playing the radio in the bedroom, selected group, added the kitchen and hey presto it is working, went to ungroup the bedroom and it stopped working.  the kitchen is still showing in my rooms.

Getting frustrated (two days been at this)

If you are still using the Bridge, be aware it’s power module can go bad which can break connections intermittently. You could either cable connect one of your devices to your router or find a second hand Sonos Boost to replace the Bridge, or switch the system over to your WiFi.

Bizarrely working this am.  so don't want to interfere with it, but if it goes down again will start with the Bridge.  Thanks.