Same thing happened to me but rebooting my Mac Mini that Roon Server is installed on did the trick. I think I quit the Roon app first before rebooting and then opened it after and everything was as it should be. Best of luck.
Server: MacMini Intel running macOS 15.1.1 with no active audio devices directly attached. Endpoints: Two LAN-attached RoPieee devices connected to Schiit Bifrost DACs, one Apple HomePod, one MacBook Pro connected to Schiit Lyr MB headphone amp. Various smartphones and tablets. The only options shown in Roon for the audio devices list are the MacMini speakers and default audio device.
My Roon server has a long and prestigious history of dropping / renaming audio devices. But now they’re all just gone.
Ok, so this is fun. I’ve rebooted a few times with no effect, then I tripped over another post that mentioned going into System Settings in macOS and toggling the Roon and Roon Server options under Privacy & Security → Local Network. That didn’t work right away, so I thought I’d try rebooting and that didn’t work either, but rebooting a few more times with a few more toggles of those settings did work.