If you are using elementary OS, some times, you may encounter this issue such as Login Screen wont succeed once you install wing panel slim or customize it.
This is because packages pantheon-shell, pantheon-xession will be removed once you install wing panel slim. One way to fix this is to re-install pantheon-shell using apt-get via terminal / text-mode . Here is another work around need to be done, since there is no GUI, you will have to connect internet through text-mode. If you are using a DSL kind of all connected Internet, this wont be a problem, but if you are using a 3G internet USB gadget, it will be an issue, like me.
So, what to do ? well, there is a command line version of network-manager which is an excellent gui software for managing internet connection.
fire this command such as
this will display available internet connection in the system with UUID
#nmcli con up uudi <idnumberhere>
this will connect to internet, then you can use apt-get for install the packages.. Voila..