Have you checked that your WiFi meets the Sonos Network Requirements FAQ? My uneducated guess would be a change in the security setting on the network, but it could be as mundane as wifi interference.

I have all my other Sonos products all working happily and have also moved the Sonos 5 around but I still just get ‘Not connected’ in the app. I’ve done all the reboot suggestions (product and router). It’s on the network ok and I can play through it when it’s physically plugged into the router

Since there is no Sonos 5. (Yes Sonos marketing made this mess)

Do you have a Five or a Play 5? If the Play 5, which generation?

It might be it is old and not able to use your newer network settings while your other Sonos can.

The Sonos support FAQ has that listed, search on wifi. 

Are any of your other SONOS units wired to the network? Is 2.4GHz WiFi available to the PLAY:5 (Gen 1)?

Try wiring one of the other SONOS units to the network.

Thanks - it’s a Play 5 and I’ll give that a try

I been having same issue for past 2 years since they screwed up the update. I wasted over $5k in their product which are all bricks now. I have reset, rebooted, added in Sonos Boost and still not working. They have become a terrible brand with zero support or care in improving. Sell it you can