It sounds as though your Roon software has been updated to release 2.0.25 - which was released in November.
With this release, the Roon software has been split into two applications: Roon (the user interface) and Roon Server (the “brains” of Roon), and it’s possible that the Roon Server application on your Mac is not running. You need to have it running first, and then Roon (the UI application) should connect to it.
Please take some time to read the Release Notes to understand the changes that have been made, and then if you still cannot get started, read the troubleshooting tips. If they don’t get you up and running, please come back here and the Support team will help you further.