What happens if you click the Select a different Roon Server button? Ignore the warning that it will show. It will then search for all available servers on your network and most likely it will find your usual one.
You should never do that because it can lead to Roon database corruption. Either do a short press on the NUC‘s power button, which will shut it down cleanly, or go the ROCK web admin interface in a browser and click the shutdown button there.
It is also possible to hear people say that the earth is flat. People say a lot of things. There is no known technical mechanism for how this could happen over Ethernet.