It depends on what exactly you mean by "freeze", but generally, if X (the "graphical mode") does not react anymore, a good way to kill some programs (or the program that's causing the freeze) is to do the following:

1. Use the key combination Ctrl+Alt+F1 to get into the non-graphical pure command-line mode.


2. Log in.


3. Use the command killall programname to kill the program you want to kill. For instance, type killall firefox. If you are unsure what the program name is, you can use the command ps aux to examine the list of currently running programs. You can also restart the X server altogether by using the command sudo service lightdm restart.


4. (Optionally) log out again by typing exit or using the key combination Ctrl+D.


5. Use Ctrl+Alt+F7 to get back into X.