Need help with Roon, system output is not showing by Streamer/DAC

I can’t play digital music thru my 2-channel system.  My Roon server is my iMac desktop computer.  When I click on the "system output" (on the bottom right of the Roon window), it no longer shows my Metrum Ambre Streamer.  A couple of days ago, I restarted my computer, and it did bring up by Metrum streamer.  Now it doesn’t work.  I not a computer whiz, so I am at a loss.  Meanwhile, I have no music thru my 2-channel system.

My system is Metrum Ambre streamer (on LAN with ethernet cable), Metrum Onyx DAC, Atma-Sphere MP-3 preamp, Atma-Sphere Class D amps, Acoustic Zen Crescendo Mk II speakers.

Please help.  Thanks!!