you should take a walk. it works.

First, you need to think about what made you sad. Say to yourself, "Is this really worth to feel sad about it?" What I mean, it's all about your will and your convection. In my opinion, nothing ever is worth to feel sad about it. Try to do things that make you happy. Watch a funny movie, listen to music. And since you're a Muslim(I guess), the best thing to do is to ask Allah to take way your sorrow.