This will occur each time you power off and reboot your Mac
Go to settings/privacy&security/local network and toggle OFF and back ON the Audirvāna entry. Then go to Audirvana and refresh the network (circular arrows). Your Hegel should re-appear. Does for me every time.
For those of us who experience this issue it is required after almost every re-start. For some reason with affected configurations the macOS local network device access permission seems to freeze/crash on a reboot. Doing the toggle forces the system to be restarted followed by refreshing the network devices in Audirvana restarts access until the next reboot. This occurs for me almost every reboot.