按以下方法解决
boot your system with LIVE CD or Pendrive
open terminal and execute sudo fsck /dev/sda1 (sda1 is generally used, if you have a different drive on which ubuntu is installed then give that)
It may prompt you several times asking "y or n", in that case keep pressing Y.
after process is over , reboot the system and remove the pendrive

启动你的live系统，在live系统里打开终端，输入指令
sudo fsck /dev/sda1 (注意，这里的sda1是系统根目录，你要输入自己的根目录，不一定也是sda1)
然后回车，会提示你按y键来确定，每次都按y,直到结束，然后重启系统即可。