Could you now check in the Main-Tab (which is closed now) if you didn´t turn down the volume by accident.
Next step would be to check the plugins in the inserts. I would try to bypass or remove them. it could easily be that one of those plugins isn’t passing sound.
It could be MCompare when a slot with no audiofile or missing audiofile is selected.
Happend to me once when using ADPTR Metric AB.