Sometime in case of root password lost or file system corruption, you need to boot system with simple shell, skipping the init sequence.
1. At Grub prompt press ‘e’ to edit command before booting.
2. Select ‘kernel’ line
3. Press ‘e’ again to edit selected command
4. Type follow at end of the line: init=/bin/bash (or sh)
5. Press ‘b’ to boot system
6. Now you are at shell prompt. Enjoy