how to recover root password
1. reboot system
under GRUB, press ‘e’, choose kernel, press ‘e’ again, choose the kernerl line and press ‘e’ to edit it. in the final of the line, press ‘space’, and type in ‘1’ (‘single’ or ‘s’ can also work here).
press enter, then press b to boot
2. after boot , passwd, change the password, reboot then ok.