You can to boot single-user mode. If the system boots, but does not log in when it has completed booting, you should try single-user mode.
If you are using GRUB, use the following steps to boot from single-user mode:
If you have a GRUB password configured, type p and enter the password.
Select CentOS/RedHat Linux with the version of the kernel that you wish to boot and type e for edit. You will be presented with a list of items in the configuration file for the title you just selected.
Than Select the line that starts with kernel and than type e to edit that line.
Than Go to the end of the line and type single as a separate word (press the button Spacebar and then type single). Press [Enter] to exit edit mode.
Back to the GRUB screen, type b to boot into single user mode.
# If you are using LILO, specify one of these options at the LILO boot prompt (if you are using the graphical LILO, you should press [Ctrl]-[x] to exit the graphical screen and go to the boot: prompt):
boot: linux single
boot: linux emergency