The way to reset the root password on centos7 is totally different to Centos 6. Let me show you how to reset root password in CentOS 7.
1 – In the boot grub menu select option to edit.
2 – Select Option to edit (e).
3 – Go to the line of Linux 16 and change ro with rw init=/sysroot/bin/sh.
4 – Now press Control+x to start on single user mode.
5 – Now access the system with this command.
6 – Reset the password.
7 – Update selinux information
8 – Exit chroot
9 – Reboot your system