Hi and welcome. Usually a baby refuses one side because she cannot latch well on that side. Other reasons might be if milk flow is super intense on that one side. Another reason might be if baby cannot get milk out of that side for some reason or another.
If a baby is refusing to nurse or cannot latch, even if only on one side, that is a pretty serious issue. If you are able to see a board certified lactation consultant (IBCLC) I would suggest do so without delay. If you saw one in the hospital or more than a few days ago it is time to see them again or see another. Here is info about what to expect at a lactation appt. http://www.cwgenna.com/lconsult.html
If you want us to try to help, it would help to know more. How is baby gaining? How many times in 24 hours does baby nurse? is baby supplemented at all? Do you feel full or uncomfortable on the left? Are you expressing or pumping milk from that side?
Did baby ever latch on the left? If yes, when did baby stop? What have your tried? and anything else you think might shed light on the issue.