If you have lost or forgot root password of RHEL7, you can follow this step to change to a new password, this is different with the RHEL6 single-usermode,
1, press e button in GRUB 2 boot screen.
2, at the end of Linux line add init=/bin/sh and remove 'rhgb' and 'quiet'
3, Ctrl+x to boot the system with the parameter
4, remount the file system as writable, run the mount -o remount, rw / command
5, passwd to change password.
6, make sure SELinux context of the files that were modified is restored, touch /.autorelabel
7, last step, resume the initialization and finish the system boot. exec /sbin/init
Redhat official link to this: https://access.redhat.com/documentation/en-US/Red_Hat_Enterprise_Linux/7/html/System_Administrators_Guide/sec-Terminal_Menu_Editing_During_Boot.html#sec-Booting_to_Rescue_Mode
