I’m posting this because of the repeated problems I’ve had my Sonos network and components working then falling off of my system. This is potentially a fix for many people out there. 
 

I have an Asus router, but many routers will use the 2.4Ghz and 5Ghz bands simultaneously WITH THE SAME SSID NAME. This will confuse your Sonos system, as well as many other WiFi connected devices.
 

For me, my Arc would connect to the 2.4Ghz band and my 2 Era 100’s would connect to the 5Ghz band. While the components were all connected to the router, the fact that they were on different bands did not allow the Sonos network to connect to all 3 of them. What’s more infuriating was that if all 3 coincidentally connected to the same band and were set up on a Sonos network, the router may move 1 or 2 to another band at a later time and the Sonos network would show those components as disconnected or missing or in Sleep Mode or powered down. 
 

THE FIX:

Find the WiFi settings on your router that “combine” your 2.4Ghz and 5Ghz bands under the same SSID name (Asus routers call it “Smart Connect” or “Dual-Band Smart Connect). Change them to different names and set up your Sonos system on one of the bands. Everything may magically start working like it did for me. 

I really hope this helps a lot of people out there that have fought with their Sonos connections.