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, maybe you forgot your password or for some other reason you need to break in or fix your system? It could also be because you can't mount your root / or some other /etc/fstab error and many other errors.
1. Edit your grub settings and find the linux16 /vmlinuz and then the "ro" portion.
2. Replace "ro" with "rw init=/sysroot/bin/sh" and boot.
3. Type chroot /sysroot to enter your OS environment to fix/change things.
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