There are many reasons why a program might stop responding. When this happens you will have to restart your program.
Unfortunately, the program is frozen and the close button will not respond. Follow the steps below to unfreeze your application.
Press 'Ctrl + Alt + End'.
Press 'Task Manager'.
Select the frozen application and then click 'End Task'.
Below is an example on closing 'Microsoft Excel'
In your case you must select the program that is frozen and not responding.
This will close your application.
You can now restart your application.