This may happen because you changed the user interface to Qt4 in Xubuntu. To restore:
1.Press Alt-F2, then enter system-settings-show.
2.There, change the user interface other than Qt4.
3.Restart X by pressing Ctrl-Alt-Backspace.
Or, the config files for xfce4 is corrupted:
1.Open your file manager->View->show hidden files.
2.Go to .config, then delete xfce4.
3.Restart X. That's it!
Note: It will reset the desktop configurations you've made. It is best if you copy it before deleting.