For Ubuntu 14.04.1, this problem can be solved by following steps.
1, Switch to console mode
CTRL + ALT + F1
2, Stop lightdm service
sudo service lightdm stop
3, Generate Xorg new configure file:
sudo X -configure
4, Move the Xorg configure file to system dir:
sudo mv xorg.conf.new /etc/X11/xorg.conf
5, Open '/etc/X11/xorg.conf' and substitute 'mga' to be 'vesa'
6, Start lightdm again with new configure file
sudo start lightdm
For Ubuntu 16.04, this problem seems to be there and cannot solve by above way.