Thanks much, Tuntes! That worked!
A few notes for people like me who are unfamiliar with all this:
* To get to GNU GRUB, you may have to go through the "
Select boot device on startup" process, then reboot, then Continue, then press Escape a few times. You end up on a page that says something about GNU GRUB at the top, and has a prompt like grub>
* Do `ls` to see what the partitions are. Then to look at a given partition, type (for example) `ls (hd0)/`.
* Ubuntu may be inside a boot directory in the partition. So you may need to type (for example) `ls (hd0,gpt1)/boot/` in order to see the ubuntu directory.
* If Ubuntu is in a boot directory, you need to include that boot directory in the path for the next couple of commands. Like: `linux /boot/vmlinuz root=/dev/sda1 ro single` and `initrd /boot/initrd.img`.
* There are restrictions on what you can set the new password to be. For example, you can't set it to "parallels" or to a string less than 8 characters long.