In trying to restore the default login screen, I learned that LightDM is responsible for running the display manager which starts the X servers, user sessions and greeter (which is the login screen). The one I'm missing is the Unity Greeter, which is default in Ubuntu 14.04.
Some forums suggested to reconfigure LightDM with
~$ sudo dpkg-reconfigure lightdm
and set lightdm to be the default DM.
I did this and did a logout, but it changed nothing.
Then I logged in again, and tried out the following (user configure LightDM), but then I rebooted the machine, and the the unity login screen was back, and then I reversed the changes I made in the LightDM myconfig, and rebooted again, and the unity login screen was still there, so apparently the LightDM reconfigure solved the problem for me.
As I learned from LightDM Ubuntu Wiki page, LightDM configuration can be overridden with user configuration.
To open the example file showing all the possible configuration:
~$ gedit /usr/share/doc/lightdm/lightdm.conf.gz
To create the user conf file:
~$ sudo mkdir /etc/lightdm/lightdm.conf.d ~$ sudo touch /etc/lightdm/lightdm.conf.d/50-myconfig.conf
To edit the user conf file:
~$ sudo gedit /etc/lightdm/lightdm.conf.d/50-myconfig.conf
I just copied the example file content in gedit my config file and edited the relevant section.
To edit the greeter, you'll have to edit the "greeter-session" in "[SeatDefaults]" section.
in [SeatDefaults] section, there is a line
uncomment this line by removing the #, and change "example-gtk-gnome" to one of the greeters available to you.
You can check available greeters by
~$ ls /usr/share/xgreeters/
greeters have an extension .desktop, their name is the one before the extension.
To use unity-greeter, you'll need the line
to be in your user config file.
Restart your computer to try it out, if logging out does not show any changes.