Not sure. I’ve moved my library off my Mac and on to an NAS, which hasn’t done any “disappearing” act, since it’s playing as I type this. 

I do wonder if there’s a possibility that you have a mesh network of some kind, and the part of the network that your computer was connected to changed in some fashion….although I’d expect that to effect the ability to reach your Sonos completely, not just make the library setting disappear.

I did have a firmware update the other night (two days ago, maybe?) that required a re-scan of my library, something I’d not seen before, but it completed in under 2 minutes, so I hadn’t really given it much thought. 

Any chance that you submitted a diagnostic that Sonos could have looked at to potentially see the event that disconnected the path to the library?

I suppose the converse thought would be: Do you have anyone in your household that might be running another controller that may have accidentally removed that path from the controller? Not that it’ particularly in a place that is easy to find, but I guess it needs to be asked. :)

Thanks for the replies. No update and only me and my MacBook and iPhone. 
ok will see how things go from now. 
cheers