First of all, ascertain if your teen's depression is normal adolescent moodiness, a result of pressure at school, social rejection from a peer group, a breakup with a boyfriend/girlfriend or due to loss of a family member or friend. If the depression has a cause, it should dissipate within a week with plenty of empathetic listening and understanding. If, however, your teen's depression lasts longer than two weeks or is accompanied with talk of suicide or death, drug or alcohol use, refusal to go to school or see friends, extreme sleep disturbances or changes in appetite, or self-destructive behavior, it's time to get professional help, as these could be signs of clinical depression or bipolar disorder.
Both clinical depression and bipolar disorder are treatable illnesses, but they should be diagnosed by a doctor. Don't tell your teens to snap out of it, count their blessings, or to "just act normal." While the moral support that you provide for your teens is of great importance, unless you're a psychiatrist, it probably won't be enough, as your teen may need medication. Until the medication "kicks in," which could take a few weeks, you might have to supervise your teen to make sure that he or she doesn't crash the car or do something else that could cause bodily harm to her or someone else. If you are unable to provide supervision for your teens who really need it, you can have them admitted to the psychiatric department of the hospital, but that will probably be a last resort. Try to arrange your work schedule or ask if you can go on sick leave to care for your teen at home.
You can take your teen shopping, go on walks, listen to music together, involve your teen in crafts projects or get your teen to get a physical work-out while you wait for the medication to start working. If your teen's friends are understanding and optimistic, encourage them to come by to visit; but if they'll criticize your teen, tell others about what your teen's going through, or are depressed themselves, discourage such visits.