In Centos 7 the days of editing the "kernel" line and adding "single" are gone. On top of that sometimes after a new install passwords do not work (possibly because requirements were not met but the installer never mentioned this?).
1.) On bootup edit the bootline by entering GRUB.
Find the line that says "linux16 /vmlinuz"
Edit the part that says "ro" and change with "rw init=/sysroot/bin/sh