The great thing about Linux systems? They're so highly configurable. The really rough part of Linux? When things go wrong, it's hard to figure out where to look. To get your GNOME-based desktop back to its startup condition, try this terminal command.
When the desktop, toolbars, file manager or any other desktop-related settings seem to have gone astray, you can use this nuclear option terminal command to get things back the way they once were, as pointed out by Linux Journal:
rm -rf .gnome .gnome2 .gconf .gconfd .metacity
It will wipe out everything you've done to customise your system desktop, but it also gets things back to working order, which would likely take you longer than making your customisation tweaks again. Hit the link for a deeper explanation of what you're doing, and why, at Linux Journal.
Resetting Gnome's Settings in Ubuntu [Linux Journal]