Hi @Mark_Allison,
Stopping Roon Server is similar, but a little different. You can stop Roon Server from the taskbar on your iMac - here’s an image on how to do so (find the Roon jellyfish on your upper task bar and click it):
If you stop Roon Server and restart it, does it fix the issue you’re seeing?
It’s worth noting that Roon Server may also be stopped when your iMac is put into sleep mode. Here is more information on the community you’ll find helpful: