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.
- In the boot grub menu select option to edit.

- Select Option to edit (e).

- Go to the line of Linux 16 and change ro with rw init=/sysroot/bin/sh.

- Now press Control+x to start on single user mode.

- Now access the system with this command.
chroot /sysroot
- Reset the password.
passwd root
- Update selinux information
touch /.autorelabel
- Exit chroot
exit
- Reboot your system
reboot
That’s It. Enjoy!