You're having a fight with a close friend. Or maybe school's been getting you down. Or you're just sad. The point is: sometimes you're sad and that's okay.
As a society, we tend to frown upon sadness and view it as something we should just brush away. This shouldn't be the case. Sadness is there for a reason and you can learn from it. Take the time you're sad as a teaching moment to uncover the source of your sadness, how you can resolve the situation, and how to stop it from being repeated. In this way, you make the most of a bad situation and can, hopefully, save yourself from experiencing it in the future.
Don't force yourself to be happy. Don't let your friends force you into being happy. Most things take time to process and during this time you should let yourself be sad. You don't need to go out on the town. Sure, if you distract yourself you will probably feel better, but that is purely temporary. Distractions are like a drug; You use them and feel better, but at the end of the day and when they're gone, you will just be even more sad.
Don't pretend to be okay. We all know how to fake a smile and respond to the question, "How are you?" with a pleasant, "I'm fine" or "I'm okay." You're not only lying to someone, but you are also lying to yourself and making yourself feel more isolated from the world. Be honest with yourself and your loved ones that in this very moment you are sad. You are sad but you are probably as okay as you could be doing in the situation.
Hang out in your sweats, listen to sad music, and let yourself be sad. And maybe in a little while your sad music won't seem so sad and you'll be on the way to happy.