You edited /etc/shadow. That is normal to remove the root password, however, you need to check again the state that you have left this file in.
Boot from cdrom and check again /etc/shadow. Ensure that the root user information is the first line of the file. If you have accidentally inserted a blank line, or any other character, at the beginning of the file, that will cause what you are seeing.
mount the ISO of the install media, change the boot order so that the CDROM is hit first and then follow the prompts to reset the password DON'T reinstall the OS.