Take the following steps:
2. Next, you will see grub-boot loader screen. Select Recovery mode and the version of the kernel that you wish to boot and type 'e' for edit. Select the line that starts with 'kernel' and type 'e' to edit the line.
3. Go to the end of the line and type init=/bin/bash as a separate one word (press the spacebar and then type init=/bin/bash). Press enter key to exit the edit mode.
4. You will be back at the GRUB screen, type 'b' to boot into single user mode. This causes the system to boot the kernel and run /bin/bash instead of its standard init. It will allow us to gain root privileges (w/o password) and a root shell.
Note : This help is for breaking into system for fixing some boot issue , disk issue etc.