If the root password has been forgotten and you are no longer able to login, it can be reset while in front of the machine. The machine has to be rebooted, and you have to login in single user mode.
If you are running the GUI, click the Reboot button and let the machine reboot.
If you are running the command line, press   CTRL, ALT and DELETE and the machine will start rebooting.

When you see the CentOS Logo, with the count-down timer, you must follow these steps.
1- press  "  e "   for edit
2- You will see 3 lines of text, use your down arrow, and highlight the middle line
3- press  " e  " for edit again. The cursor will be flashing at the end of that line
4- leave a space and type    init  1
5- press ENTER  once
6- press  " b "  to boot
7- The machine will boot, and then stop with a prompt like this
    bash#
8- you can now use the normal  passwd command to reset the password
9- once it is done, just type   reboot and let it reboot.