Are you having a problem and don’t have a bootable CentOS DVD and no PIX server to get into rescue mode.  Then try single user mode.

In single user mode, your computer boots to runlevel 1.  Your local file systems are mounted, but your network is not activated.  You’ll have a usable shell to perform maintenance, but the most important thing your file systems must be mounted successfully.

At the Grub2 screen during boot, press any key to get the Grub2 interactive menu.

Select the version of Linux you would normally boot, then press edit.

Then select the kernel boot line, and go to end of the line, and press space, and add single

... single

Now press enter to exit edit mode, and b to boot.

That will boot you into single user mode.

This time, I needed to mount a disk that was not mounting correctly so Single user mode couldn’t help me.  I connected a cable back to a spare server I keep available just for doing PXE.

Performed a PXE boot on a CentOS 7 install image, and ran rescue mode, then afterwards got the mount corrected.