Unresponsive applications and system freezes are an inevitable part of computing. Unfortunately, there are only a few options to remedy these situations: you can wait for the unresponsive program to respond, or you can perform a "Force Quit." With the Mac OS, forcing a quit ends the application that's frozen so you can restart the program, or reboot your computer. You can complete a "Force Quit" from the menu bar of your iMac.
Click on the small Apple icon in the top left corner of the menu bar. Select "Force Quit" from the list of drop-down options. The Force Quit Applications dialog opens with a list of programs currently running.
Select the program you want to quit, then click the "Force Quit" button at the bottom of the window.
Press and hold the "Command" and "Option" keys on your keyboard, then press the "Esc" key, if the frozen program prevents Step 1 from working. Choose an application from the list that appears, as in Step 2, and click "Force Quit."