Depression is the most common emotional issue people have to deal with. You can be depressed and not even be aware of it. The best way to overcome depression is to catch it early and take the steps necessary to limiting the affects.
Symptoms: Low energy/lack of motivation, change in sleep pattern (sleeping more or less), change in appetite (eating more or less), feeling negative or hopeless, and avoiding social situations/isolating.
Steps to overcoming depression: Get moving/ increase your activity, eat healthy, and get out and be with people and avoid isolating. I also suggest you keep a journal and write about your thoughts and feelings. This way you can more easily identify the issues that are troubling you. Changing how you see things goes a long way toward improving mood.
You can get out of your funk and begin enjoying life again without relying on medication. If your depression persists, you may have a medical issue. Consult your doctor.