When you are feeling down if somebody cheers you up it's great, but at times that also doesn't work. Sometimes our minds behave in a complex way and feeling down is a manifestation of that complexity. At that particular moment, you may not be able to fully analyze the reason for the feeling. Later on, when you are fine, give it a thought. Then hopefully you will be able to find out the reason of feeling down.
All of us need to remember that life goes on even at times of difficulties. Keep yourself busy with activities that you enjoy. If you wish to express your feelings to a close friend call her/him up. If they are not available, sit quietly with your eyes closed as long as you can. Take a couple of deep breaths and think you have to move forward. This may work well for you.
One important suggestion to everyone. At times, we all face this situation of feeling down and it automatically goes away. But if it is too frequent, take help of a psychologist. Let the psychologist analyze why it is recurring at regular intervals.
"Life is easier when you enjoy what you do"