I have been haunted by this issue with Cubase (Pro 10.0.30) that about half of the time I open the program I’m not able to get any sound from tracks. On my system this can be easily replicated by creating a new project and then adding for example a Retrologue synth. When I either use the Retrologue on-screen keys or a midi-keyboard there won’t be any kind of sound. I can see Cubase receiving MIDI from global MIDI IN Activity indicator. When I restart Cubase it everything is (sometimes) fine again and the sound plays correctly. After the next restart it may work or not again. For me this is very weird issue and I haven’t had problems like this in previous versions. I’ve been using Cubase since version 3 or so.