I have a somewhat similar problem here, although using Cubase 9.5 now.
When I leave cubase OR nuendo idle for a few hours, I suddenly lose the output signals.
As in, when I press play, the meters do not show anything at all. Everything is quiet. Even my browser wouldn’t be able to playback youtube videos.
I think it has something to do with my Fireface UCX USB. And btw, I’ve already put on the Steinberg Power Scheme, so I’m not sure if it’s the “selective usb suspend” problem thing.
Any suggestions?