Ubuntu starting in low graphics mode?

If you ever encountered Ubuntu starts in low graphics mode, it could be due to the following:

1. your "/" or "/boot" partition are full. Do a check with df -h, if it is the case then clean out some useless stuff and do a reboot
2. the lightdm or gdm is corrupted. Uninstall it completely with sudo apt-get purge gdm or sudo apt-get purge lightgdm, then reinstall with sudo apt-get install gdm. Select lightdm or gdm from the menu. (you might try lightdm first, if it still give problem, go with gdm)
3. the desktop might have corrupted. If you are using XFCE4 like I do, continue with sudo apt-get purge xubuntu-desktop and sudo apt-get install xubuntu-desktop. Same method work for ubuntu-desktop.