It sounds like there might be corruption in the /etc/passwd file. (for example, the first line might be a blank, the line for root's entry might start with a space or another character, etc.)
See if you can get into single user mode again. When you are in single user mode, you need to use the commands that are in /bin (and /sbin) because /usr most likely will be unmounted. So if you would type /bin/vi /etc/passwd, you should be able to take a look at the /etc/passwd file and see if there is any corruption in there. You also could mount /usr (/sbin/mount /usr) and use the vipw command, which should prevent any more corruption of /etc/passwd. (It won't be able to fix any corruption that is in there already, but perhaps you will be able to see where any corruption is and fix it.)
Make a copy of the /etc/passwd file and then you could try deleting the encrypted password (the second field in /etc/passwd entries -- between the first and second colon) for root, boot all the way up and log on as root. You won't be prompted for a password, but you can set it with the passwd root command.
