Go into recovery mode to rescue system hang in Ubuntu 12.04

In the bootup, press and hold left-shift key, the grub menu will show up. Select the recovery mode and drop into the root shell. The root file system is mounted as read-only in this time. We need to invoke below command to re-mount it as read-write permission.

mount -o remount,rw /

Ref: https://wiki.ubuntu.com/RecoveryMode