Re: Depression?
Besides SAD as mentioned by Ask99 - what you can do is talk to her and bring her back to the traumatic event that has caused this. Let her slowly speak her mind out about what frustrates her. Let her talk about what could she have done differently and how things would have been different if this event had not occurred. Let her empty her soul out, let her cry and relive that moment. Allow her a chance to express herself. you just listen to her quietly. Take her out for a walk. Slowly but steadily get to know what makes her smile - and take it from there.
Resolving this will not happen overnight. I know it's not as easy as I made it out to be.