This solution doesn't work. I'm using Guest account right now.
I was just working on VirtualBox and listening to some music with Rhythmbox when suddenly my computer became laggy, and seconds later, everything disappeared except my cursor and wallpaper. I ran terminal using CTRL+ALT+T and typed
sudo startx, and the last one
sudo shutdown -h now. After booting up my computer again I could not login to my user account, after typing my password and hitting enter, black screen for 2 seconds, and login screen again.