Your ubuntu desktop crash or you make some changes in Ubuntu Desktop that require restarting the X Server. To restart the Xserver without rebooting we usually use the key combination Ctrl + Alt + Backspace. but, Since Ubuntu 9.04, this key combination has been disabled by default, to eliminate the problem of accidentally triggering the restart.

To Enable key combination Ctrl + Alt + Backspace in ubuntu 12.04 LTS, if you are using unity go to Unity dash type “Keyboard Layout” or System Setting -> Keyboard Layout -> Options expand “Key Sequence to Kill X Server” then Check list

Done, now you can restart X server Ubuntu 12.04 with key combination Ctrl + Alt + Backspace without rebooting.

To restart Ubuntu X Server without rebooting, you can also use following commands:

sudo restart lightdm

or

sudo service lightdm restart

by execute command above, Your desktop will disappear for a few moments and it will reload back, if you are still using Ubuntu 11.04 or older, run command below on terminal to restart x server:

sudo restart gdm

or

sudo service gdm restart

This command may also work for all Ubuntu versions:

sudo pkill X