Q: Ubuntu 15.04 boot hangs on “Starting light display manager” after updates
A: Try to go to tty1(Ctrl+Alt+F1)
Login and purgexorg
andlightdm
sudo apt-get purge xorg lightdm
Autoremove whatever's left
sudo apt-get autoremove
Install lightdm and xorg
sudo apt-get install xorg lightdm
Q: Debian jessie boot hangs at “Started Update UTMP about System Runlevel Changes.” A: There is probably a problem with your video driver. I resolved it by following (modified) instructions at http://ubuntuforums.org/showthread.php?t=2072420:
Press Alt+F2 to switch to a new console sudo apt-get purge xserver-xorg-video-intel then reboot sudo apt-get install xserver-xorg-video-intel nano /etc/X11/xorg.conf, remove any present code (if applicable) and enter the following:
Section "Device"
Identifier "Card0"
Driver "intel"
Option "AccelMethod" "sna"
EndSection
Save the file and reboot