If you cannot log in as root for any reason, you can always boot into single-user mode. How this is done is architecture dependent. If you are using one of the architectures that has a second-stage bootloader, such as i386 or amd64, you can boot into single-user mode by typing in "-s" in response to the "boot>" prompt.
In single-user mode, the root filesystem is mounted read-only. No other filesystems are mounted, and no services or network have been started. You can mount your filesystems read write:
# mount -a
then set a terminal type:
# export TERM=vt220
and make any corrections you need to make. What you need to do to fix the problem will depend on what you did. Do you just need to change the root password? Do you need to repair an error you made editing a configuration file such as /etc/ttys?