A GUI is a very complex piece of software and there could be any number of reasons why it would lock up on you. If you need to kill your x server and then restart it, here are some tips:
Killing X server via Command
Type “init 3″ in your terminal and that should kill x server. If it doesn’t find the command, then switch to the root user by typing “su -” (you will need to know the root password).
Restarting X server
Simply use the keyboard combination CTRL+ALT+BACKSPACE. Some times this will help, but this combination will explicitly restart the X server.
Working with a Console
If the previous keyboard combination did not work then try CTRL+ALT+F1. This will not kill X server, it will simply open a full screen console. I believe Red Hat and Fedora Core have F1-F6 as options. If you want to switch back to a GUI environment use the keyboard combination CTRL+ALT+F7.
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