you need to go see a doctor, there are lots of things that can help like group therapy or medication
I disagree with this previous answer, especially if you are a teenager or college student. All of us have passed through a stage of disillusion, dissatisfaction, a feeling of being not understood, and finally the impression of being nonexistent or worthless. You are surely not the first one and most of us passed through that challenge. Because it's really a challenge and it's up to you to reverse such feeling asap and start to become a real man with a real objective in your mind. For example take right now the decision to help others who are in the same feeling situation as yours and commit yourselves to be the heroes who will save others from sinking in black moody blues. Get on your feet, get friends, enjoy life together and don't stay alone. That's what friends are for.