1.Power off
2.Long press shift during system starting to enter recovery mode.
3.Select Root to enter into terminal
4.cd /etc/X11
5.cp xorg.conf.failsafe xorg.conf
6.chmod 777 xorg.conf
7.sync, reboot
8.Ubuntu can boot normally, but the resolution is very low, like 800x600, or 1024x768.
9.cd /etc/X11
10.mv xorg.conf xorg.conf.bk
11.sync, reboot.
12.Ubuntu resolution become matched with your monitor.