Might be that they all rebooted and are struggling to regain their network IP address, perhaps? Try a router reboot first and if no joy, then switch off your Sonos products and bring them back online one at a time.
 

If the issue reoccurs during Sonos updates, then if not tried already, perhaps reserve (make static) the Sonos device IP addresses in your routers DHCP reservation table and see if that resolves the issue for you.

Thanks Ken

Will, when I have more time to spare, have another go.  Surely this shouldn’t be so difficult, though.

I agree, but if what I mentioned is the issue, then quite often it’s the DHCP server software within the local router that’s likely not handling multiple products rebooting at the same time. Making the Sonos IP addresses static can often resolve the issue. I hope the suggestion resolves the issue for you.