Depending on what you set “what” to?
Sounds like it could be an interface problem rather than a Cubase problem. Hard to tell without more details.
So …
Mono or stereo input?
Mono or stereo track?
What is the track’s input set to?
What is the track’s output set to?
If you are recording a mono input, set up a mono input in VST Connections and select that as your track’s input.