Sometimes it is necessary to get root access into the Ubuntu system. I was facing the issue when try to reset forgotten root password in Ubuntu 14.04 using recovery mode (Drop to root shell prompt). After done a few testing and do a research, i have found the working solution for it.
Give root password for maintenance (or type Control-D to continue)
1. First, make sure to choose the regular or default boot kernel that you always use, then press ‘e’.
2. Hit the down arrow key over to the “linux” option.
3. Remove the “ro” part with the backspace key, and then the following onto the end:
Press Ctrl-X or F10 to boot.
4. Your system should able to boot up very quickly to a command prompt.
5. Use the following command to reset your password:
Once password has been set successfully, run sync command to write out data to the disk before rebooting.
sync rebooting -f
Thats all. Now you should be able to login without any issues.