Hi there. Everybody wil feel unhappy and low at times but if feeling sad for very long time den may be your suffering from depression.
Symptoms of depression include Difficulty concentrating, remembering details, and making decisions, Fatigue and decreased energy, Feelings of helpless, hopeless and worthless, Insomnia, early morning wakefulness, or sometimes excessive sleeping, Irritability, restlessness, Loss of interest in activities which were pleasurable before, Loss of pleasure in life, Overeating or appetite loss, Persistent aches or pains, headaches or digestive problems that do not ease even with treatment. Persistent sad, anxious, or "empty" feelings, Thoughts of suicide or suicide attempts.
Treatment includes relaxation exercises, discussing to your loved ones about your problem, psychotherapy, anti depressant medications, read motivational books, quotes. There are many online groups and site where people who suffered from depression and cured, help others to overcome depression. You can join such groups which help you to speak out. Please do consult psychiatrist, attend counseling. Don't attempt any life threatening activities.