I know exactly what you mean. I went through the exact same thing my sophomore year. I went to an online school to help me with out of school activities and I got really depressed. Now its my senior year and I'm going back to my old public school because I feel like i cant keep staying home all the time.
As for your questions, You should retake the ACT so you can get the highest score possible and then take several Sat 2's. What you want as a score really depends on what college you want to go too. You can help yourself now by trying to be more involved in community service and you EC's. You will also want to explain to the admissions officer what happened during your sophomore year and why your grades got so low. Summer school can always help to redo classes and get ahead. And no, the class wont disappear if you retake it, it will stay on your transcript.
For me, I actually got diagnosed with Anxiety Disorder. You might want to talk to your doctor. Just find new ways to get out and about. I joined the Boys and Girls Club of America's Keystone program (leadership) and am now a Junior Adviser and Activities Director. Since then, my depression has really gone away.