My minecraft launcher isn't working?

First, back up your saves. To do so, click your start button, type "Run" and hit enter. Then type %appdata% and hit enter. Go into the.minecraft folder and then into saves and copy those somewhere. After backing up your saves, delete the entire.minecraft folder. Once all of that is cleared, double-click the Minecraft application icon and it will load in fresh files. You will have to sign in and copy your worlds back to the saves folder and reinstall any mods you had.