It may happen that an application may stop responding and not responding to user input...
the only remedy is to run a "kill", from a shell just run the following command:
kill -9 [pid of the application frozen]
the option -9 equals SIGKILL.
If the application locked is a graphics application, you can run:
xkill
and with the mouse simply click on the application to be close.
Note: the signal SIGKILL immediately terminates the process, which does not perform any rescue operation or discharge data.
