Just follow the simple steps given below in case you have forgotten your root password and need to reset the same :
1. At grub boot screen (after restart) , Select the Kernel.
2. Press the e key to edit the entry .
3. Select second line (the line with the word kernel).
4. Append the letter S or Single or 1(depending on the system) to the end of the (kernel) line.
5. Press ENTER key.
6. Now press the b key to boot the Linux kernel into single user mode.
7. you will be promted a root shell.
8. type passwd root to change the root password.