Reset root password

RHEL 6

Append init=/bin/bash to kernel command line at boot (single user mode might not work if it was protected by a password)

mount -o remount,rw /
passwd root

RHEL 7

Append rd.break or rw init=/sysroot/bin/sh to kernel command line at boot

mount -o remount,rw /sysroot
chroot /sysroot
passwd root
touch /.autorelabel                                  # if you use SELINUX
type exit twice -> the OS will continue to boot