no input or output connected

I’m using Cubase 8.5. and for some reason there is no sound. Recorded tracks have stopped playing and also there is no sound when recording live. Sometimes I can see the levels working (on tracks that I have managed to record) but no sound, and other times there is no level at all. In both cases, there is no sound in playback. Occasionally a window will pop up that tells me my Alesis iO4 isn’t connected which is confusing, because it is. So I reconnect everything and check in VST Connections that all inputs and outputs are set correctly and yet, nothing. Sometimes after rebooting my computer a million times it works but then for no reason it stops again. I really don’t want to have to give up using Cubase but if this continues I will have no choice. Can anyone please help?