How can I be happy again?

At school, it feels like I've lost all my friends, and I've been really weird with my sleeping patterns. I used to cut myself, and I really want to again, but I change in the locker rooms because I don't want anybody to see. I get ticked off easy and overreact. At night, I'm very depressed and listen to sad music and all that sort of stuff. Please help me. I just want to be happy again, and I feel like I'm starting to fake a smile at school.