I have seen numerous posts about restarting xserver in Ubuntu and nearly all point to Ctrl+Alt+Backspace. This command no longer works in Ubuntu since v9.04. The command has been replaced by RtAlt+SysReq+K. On my laptop the SysReq key is shared with print screen and if done in a different order produced a print screen dialog. This of course may be different for your equipment. I have also seen one post about adding the dontzap package to re-enable this function. When I ran the command sudo apt-get install dontzap I received package not found error as seen in the screenshot below. If anyone knows how to re-enable this please post in the comments.