Did you have good study skills before your bout of depression? My only thought would be that perhaps, if that is the case, you are still struggling with some symptoms. You don't permanently lose all of your study skills (those are learned skills). Can you think of anything else that might be interfering with your ability to focus? Are your classes interesting? Is there simply too much work? Are you not sleeping well? Is there just something else that you would rather be doing instead, or is it pure inability to sustain attention on anything (in which case I would go back to wondering if maybe you are still dealing with some depressive symptoms)?