Sometimes the Mac crashes... not usually OS X, but sometimes Applications themselves crash.
No problem.. just 'Force Quit' the program. Here are two ways to do it.
Use the Apple Menu
or hold down COMMAND + OPTION + ESC
Each of these will bring up the currently running Applications
Simply choose the Application that is 'Not Responding' and Click on 'Force Quit'.If the app is still respondig, the option will be to 'Relaunch' the app.
One last way to do it is to Click and Hold of the app on your Dock, and when the menu pops up, it will say 'Quit'. Simply hold down 'Option' and it will change to 'Force Quit'. Choose that option.