Sometimes Ubuntu (or any other operating system for the matter) applications just freeze and you need to shut them forcible or kill them. This is not often advised but sometimes there is no other alternative. It may happen if the application has run into an infinite loop or there is some internal error that has hung your application
If your application or process is not responding, you kill it by using the xkill command in Ubuntu. It’s very easy to execute. Press ALT+F2. You come across this window:
In the bar type xkill and click Run. The cursor turns into a small cross. Click on the window application you want kill, and it is killed, or shut down.