You might have had experience with your ubuntu system hang up due to some applications (this happens sometimes with Firefox and some games).
You can actually login to one of your other terminals and stop the applications which is causing problems.
1. Use Alt + Ctrl and any function key F1 through F7. Where F7 is the default xwindows terminal.
2. Log in and then use the command “top” from the command line to find the process id (pid) of the frozen or run away application which is causing problems.
3. After finding the pid number, type
$ kill -kill PID (where PID is the process id of the application).
Now go back to terminal 7 with “Alt+Ctrl+F7” to find the troublesome application gone.