To boot in the single user mode (rescue mode) press e (edit) on the GRUB2 boot screen. Scroll down to the line beginning with "linux16" (on x86_64 bit systems) For other systems refer RHEL 7 documentation. On the following screenshot this is underlined in red. Add the following to the end of this line (underline in green in the image below).
systemd.unit=rescue.target
Press Ctrl+x to boot the system in the rescue mode. Enter the root password when prompted to gain access to the system in single user mode.