Normally, we can restart x server with CTRL+ALT+BACKSPACE keyboard combination in an older version of Ubuntu.But in jaunty this is not possible because newer version of Xorg disabled this property.So that if we want to re-enable this feature we just have to modify Xorg.conf file,lets do that from console

sudo nano /etc/X11/xorg.conf

then add this 3 line to it

then save the file and quit.Logout and login again, it should be work.