Did you ever get sound out of both sides, or is this a new setup?
Are you using any adapters or converters that you can take out of the loop?
You need to do the simple tests.
Try another pair of headphones, even if they're just cheap earbuds.
OR
Try your headphones plugged into something else. TV, Radio, Hifi, whatever. Prove that they do or don't work.
Regardless of the outcome, check the main monitor outputs.
It doesn't really matter what kind of amp/speakers you test with. As long as you can confirm that signal is or isn't coming out of left and right.
If you end up proving that both the cans and speaker outputs are only giving one side, then the chances are it's a software issue rather than hardware.
Make sure cubase is set to output to L+R, or 1+2, whatever it's called, and make sure you don't have any pan knobs turned.
Keep those in the middle for now.