I was able to set up a text UI by editing the file /etc/default/grub in Ubuntu 12.04 as per the instructions in: How do I disable X at boot time so that the system boots in text mode? But, how do I revert to X-session as the default one? I don't want to log into text mode and 'start' the lightdm every time. So I tried replacing back the GRUB_CMDLINE_LINUX_DEFAULT="text" to GRUB_CMDLINE_LINUX_DEFAULT="quiet splash", and did update-grub too, but something unexpected happened, an unbootable system! How do I fix this?