I did get the grub screen so as you say, it was not a boot problem. I figured out it must be a problem do to with lightdm. I figured this out because after installing gdm and then running the command
and then choosing gdm, I do get the login screen.
sudo dpkg-reconfigure lightdm
In addition, before the installation of gdm, when I ran the command dpkg-reconfigure, I did not get the graphic screen giving me the option of dms. Is this only because there was one dm installed? Or is it an indication of a deeper problem? At this time, /etc/X11/default-display-manager had '/usr/sbin/lightdm' in it, which seems ok.
Although gdm is fine, I would still prefer to use lightdm. Could anyone suggest how I could fix that?