Computer “freezes” are an unfortunate but not uncommon issue faced by nearly all users at some point. Occasionally, a program may stop responding to your input, preventing you from closing the application or continuing your work. If you experience such a “freeze” on your iMac, you can attempt to force the program to shut down without having to restart your entire computer.
Press "Command," "Option," and "Escape" to open the "Force Quit" dialog box. Alternately, you can press go to the "Apple" menu at the top left of your screen and select "Force Quit."
Select the name of the frozen application from the dialog box if it isn't selected by default.
Click "Force Quit" to shut down that specific application without affecting your other open applications.