Resetting Gnome's Settings in Ubuntu
One of the great things about Linux is how easy it is to customize. Now unfortunately if you don't know what you're doing and sometimes even if you do know what you're doing, you can get into trouble.
I found this method to be very useful in times of stress. As you all may know everything in Linux is a file, all of the settings are files. Inside your Home folder is where all of the settings and user specific files are located. Remove these folders to reset Ubuntu/Gnome back to its default.
If you don’t have access to your graphical (GUI) desktop to delete these folders in Nautilus or you’re just stuck at the login screen, drop into command-line by pressing CTRL + ALT + F1, login to your account, and run this command:
rm -rf .gnome .gnome2 .gconf .gconfd .metacity
Return to your GUI desktop by pressing CTRL + ALT + F7
Now you should be back to normal.
Please note this will only reset Gnome-specific settings. If you are having issues with your video card, x-server, etc., this WILL NOT fix those issues.