1. After rebooting our system, hold on to first option i.e. default on the grub menu (Ubuntu, Linux 2.-- Generic) and then Press "e" to edit.
2. Look out for line that contains "ro quiet splash" and move the cursor using arrow keys and go to that line, simply replace "ro quiet splash" with "rw init=/bin/bash" without double quotes and then use CTRL + X to boot the system.
Now your system will boot to root shell.
In command line Type
3. passwd username //command to reset your password.
Type your new password for the username and then press enter.
4. Type Sync
5. Type reboot -f //command to reboot the system
