Some days the world is quite simply against you, but there's no need to accept that misery is your lot.
Even if you can't change the thing that's making you unhappy, you can try to perk yourself up in spite of it. But are you really sure you can't change it? If it's an ongoing problem, what's making today worse? Doing something that can strike at the root of the issue will be satisfying and make you feel empowered, even if you can't wholly resolve it at the moment.
Assuming, though, that the cause of your sorrow is unassailable, then try to find a way to let it go for a while. Distract yourself with an engrossing activity, think about the things that you enjoy and are grateful for, and maybe go get a change of scenery to encourage a new mindset. If you're having trouble breaking out of your funk, have a good winge to a friend. Letting your frustration out can help you let it go.
Getting exercise is a good way to improve your health and boost your mood. Don't believe me? Let our Why Excercise Makes You Happy infographic give you the low down.