Reson for prolonged phases of sadness.

Since a pretty long time, I have been having extreme phases of sad moods for no reason at all. It has become as frequent as alternate days now. One moment I am happy and all of a sudden something switches and I go numb. It becomes really hard to react or talk. Recently, I have been facing another problem. I feel extremely anxious for no reason at all. Sometimes it is so strong that I have breakdowns without even knowing the reason for it. Along with this, I also have issues with my sleep pattern. I have sleeping too much lately. I do not feel like getting up. I severely lack motivation to do my work. I even do not feel like going out or making any plans at all anymore. I am unable to understand the reason for my problem. Because of the same reason, I cannot figure out any solution. Kindly help me understand.