I have been trying to solve this problem on my Ubuntu 18.04 for the past two days.
I boot up my system, and it acts like the server edition of Ubuntu. It launches a full screen command line and that's it.
So far, I have tried reinstalling gdm3 and lightdm. That did not work. I set my privileges on the .Xauthority file to my username, that did not work. I also reinstalled gnome-panel as some people said and that did not work. Finally, I installed lubuntu-desktop, and set LXDM as my default desktop environment. It's still only loading the command line!
What else can I try?