MacBook Air won't restart or shut down or open any applications. When I open force quit the only thing there is iTunes and it won't let me quit that.

when I open my computer everything looks fine but I can't open any of the applications so I tried to restart and shut down the computer and it says it's going to but then nothing happens. I tried to force quit all of the applications so when i open force Quit the only thing there is finder and iTunes and when i try to force quit iTunes nothing happens there either. I try to open the app store and the systen preferences and they flash on the screen quickly then go away. Thank you!