Previously, had you been using the ONE’s as a stereo pair?
Or, possibly have them set up as two separate Sonos Households? Have you ever been able to see them at the same time in the controller?
Hi @Ferris Islander, welcome to the Sonos Community!
It does sound more like you have two separate households for the Sonos One speakers. However after looking into your system, I can see both Play:1s are part of the same household, maybe a recent change by you after posting this thread?
The Play:1 that is currently online is called Den, the other Play:1 doesn’t have a cloud connection, meaning it’s not connected to the network/system.
If you have any wireless extenders in your house, can you power them off then see if that resolves your issue?
I hope this information helps!