I booted my system this morning and found it was forced into an emergency mode like this:

I found an answer on askubuntu.com which talked about unmounting some device. This remind me of what I did last night. I configured my /etc/fstab file last night to add windows partitions’ infomations there, as well as a Seagate portable disk. But now I am in my office without that Seagate disk. Could it be the missing device that caused the booting problem?

So I booted into a live USB to comment out the Seagate device information in fstab file and reboot the computer again. Now I can boot into my system without any problem!