You need to use CentOS cd only to boot into emergency mode. You can access / in a single user mode. To fix this problem at Grub boot screen:

1. Select the kernel
2. Press the e key to edit the entry
3. Select second line (the line starting with the word kernel)
4. Press the e key to edit kernel entry so that you can append single user mode
5. Append the letter S (or word Single) to the end of the (kernel) line
6. Press ENTER key
7. Now press the b key to boot the Linux kernel into single user mode

You need to mount at least / and other partitions:
Now edit file /etc/ssh/sshd_config and allow root user. If you forgotten root password, just set new password, enter:
We have plenty of docs about grub and booting into a single user mode:

Linux: Grub Boot into single user mode



Linux reset forgotten root password


How to: Recovering Linux grub boot loader password