To restart the X Window Manager in Ubuntu 13.04, simply execute sudo restart lightdm.

Warnings:
  • doing so affectively restarts your windowing session, so you will lose any unsaved changes/data - make necessary saves/backups before restarting X
  • if you want to simply restart the Unity interface, Alt + F2 and unity --replace
  • if lightdm fails to restart, Ctrl + Alt + F1 and sudo reboot

Notes:
  • Ctrl + Alt + F1 goes into a console mode interface; Ctrl + Alt + F7 to revert back