Root password can be reset through single or rescue mode in Rehdat or Cent OS 6.
1. Reboot the server
2. Edit Grub screen while booting [after post session]. Select the kernel line, press “e” to edit the kernel. Type “S” or Single or 1 at the end of Vmlinuz line then press Enter, Press “b” to boot the modified kernel line
3. Server will boot to Single user mode and get # prompt
4. Type “passwd” on # prompt to set the new password for root account
5. Reboot the server using “reboot” command, then boot the server to multi user mode and access root account using the password.