Depression, loss of appetite and lost muscle mass. How do I increase muscle mass?

Before I returned home from college for summer break I weighed around 127 lbs, that was in May. I now weigh around 115lbs. I looked better at 127 lbs because I was solid with muscle, but due to my lack of appetite I stopped eating and I'm left with flab and no muscle tone. I look "bony" although I'm 14 lbs away from being underweight, which is good, I shouldn't be bony, but my lack of muscle makes me look that way. I was told in order to build muscle I need to eat more than my body requires to maintain my weight. But how on earth can I do that if I don't have an appetite? Is lifting weights enough to build muscle? Or do I need to eat more as well?