What to do when ubuntu freezes?
Well there in an easy way to restart your system without rebooting the machine.
To do this we will need to set-up a new shortcut.
Go to keyboard settings:
Then go to Keyboard layout, Options, Key sequence to kill X server and enable the tick as below:
Now every time your computer is not responding it is enough to press Ctrl + Alt Backspace and you will be given with the very fresh ubuntu session (particularly important when the computer is heavily loaded with penetration testing applications)