Has your GNOME desktop suddenly reverted to TWM upon a regular restart? If so, you aren’t alone.
Here is what you need to do to restore your GNOME desktop:
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1. Check if you can just get away with switching back to GNOME:
yum install switchdesk switchdesk gnome
If that tells you that gnome is not installed then proceed to step 2, otherwise you should have gotten your GNOME back.
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We are going to reinstall GNOME. Run this command:
yum groupinstall "GNOME Desktop Environment"
Now run the command to switch desktops:
switchdesk gnome
and you should be good getting your GNOME back upon next restart of your xwindow session (Ctrl+Alt+Bcksp).
Original Source from Anton
http://blog.bordercrossingmedia.com/2009/01/your-gnome-desktop-suddenly-reverted-to-twm-in-centos-5/