If you have not configured Ubuntu /Debian automatically start a graphical environment during startup. If you want to kill a process from a hanged graphical environment, you need to open a virtual console or virtual terminal.
How to start a Virtual Consoles / Terminals in ubuntu / Debian
Just press the following Keys
Alt+Ctrl+F1 (F1 to F6) . Using F1 to F6 You can open 6 Virtual Terminals.
this will ask for
Return to Graphical Screen by Pressing