The system is running in low-graphics mode after upgrading the VirtualBox Guest additions to 4.2.12

I tried switching to use gdm instead of lightdm to see whether it could help but it didn't so I rolled back to v4.2.10 while waiting for v4.2.13 which was reported that this problem has been fixed.

• Rebooted the machine in recovery mode in the Grub menu by, while booting:- 
• Pressing ESC, or
• Holding down the Shift key (this works better for me).
• Uninstalled the VirtualBox Guest additions

cd /opt/VBoxGuestAdditions-4.2.12
./uninstall.sh
reboot 

• After reboot, the screen was black. Here is what I did to fix this,
• Pressed Cmd + F1 (or Alt + F1, don't know why Ctrl isn't needed on my machine) to switch to tty1.
• Made LigthDM the default display manager

sudo dpkg-reconfigure lightdm
sudo reboot  

        But it didn't work. It turned out that I also had to modify /etc/X11/default-display-manager to have the full path to lightdm like this.

/usr/sbin/lightdm

Then rebooted,

sudo reboot

After that, I could continue installing the VirtualBox Guest additions v4.2.10 and now everything is back to normal.

Refs

https://www.virtualbox.org/ticket/11709

http://ubuntuforums.org/showthread.php?t=1186984

http://askubuntu.com/questions/172319/how-can-i-start-in-safe-mode

https://wiki.ubuntu.com/LightDM

http://askubuntu.com/questions/74551/lightdm-not-starting-on-boot