Some Unix flavors will demand a root password to reach single user mode. I know Solaris is one and I think some Linux distros are the same.
If your distro allows single user mode without a root password, you might want to edit your /boot/grub/grub.conf file (or /etc/lilo.conf if you are using lilo) after fixing the problem and create a new stanza there, essentially the same as the current one but with a new label (like "Linux single user") that already has the "single" or "1" appended. It will make it easier to run single user mode in the future.
If your distro will not let you in using single user mode without a password, then boot into rescue mode and mount your "/" partition at a convenient mount point (often the rescue mode will mount it at "/mnt/sysimage" for you.) Instead of editing /mnt/sysimage/etc/passwd
(or whatever mount point you are using) I normally edit /mnt/sysimage/etc/shadow
instead. The entry for "root" is normally the first line of the file. Remove everything between the first pair of ":"'s. For example, make:
If you can boot into single user mode without a password, then do the same editing except without the "/mnt/sysimage" part.
This will allow you to log in as "root" without a password. Once you are "root", you can put in a real root password with the "passwd" command.