If an application becomes unresponsive and “frozen”, it is sometimes helpful to “Force Quit” the application.  To do so, simultaneously press Esc+Option+Command on your keyboard

This will pull up the Force Quit Window

Screen Shot 2015-03-03 at 10.29.17 AM
You can now select which application you wish to “Relaunch”

Alternatively, you can select “Force Quit…” from the Apple Menu to quit the currently active application.

Force Quit menu