Ubuntu open to black screen with blinking underscore character after release upgrade

In Ubuntu 18.04 and later lightdm login display manager has been replaced by gdm3 and the keyboard shortcut to access the virtual console has changed from Ctrl+Alt+F1 to Ctrl+Alt+F3. sudo dpkg-reconfigure lightdm has been replaced by sudo dpkg-reconfigure gdm3. sudo service lightdm restart has been replaced by sudo service gdm3 restart.

If you know your disk is full, you can try removing around 1-2 GB of unneeded files before executing sudo service gdm3 restart.

If the problem remains after reboot fix it by opening /etc/gdm3/custom.conf for editing in nano text editor with sudo nano /etc/gdm3/custom.conf and unmcommenting #WaylandEnable=false by removing the# character at the beginning of the line as follows: WaylandEnable=false

Try the following:

press Ctrl+Alt+F1

You'll see a terminal for login, enter your credentials and execute the following command:

sudo service lightdm --full-restart

Enter your password again and you should see the login page

Here’s what worked for me, on a Dell Precision M6800 laptop running Ubuntu 16.10:

First, at the black screen with the cursor in the top-left (which wasn’t blinking in my case), I followed this answer and pressed Ctrl+Alt+F1 to show a terminal, and logged in with my username and password.

That answer suggests running sudo service lightdm --full-restart, but the command only gave me an error. Instead, this answer’s solution of removing and reinstalling lightdm was what brought me forward. Note that a wired internet connection is required for the apt-get install part; wireless internet is apparently not set up yet at this stage in the boot process.

sudo apt-get purge lightdm
sudo apt-get update
sudo apt-get install lightdm
dpkg-reconfigure lightdm
sudo reboot

After this reboot, the login screen successfully showed, at the point in time where previously the one-line log message disappeared and the underline cursor appeared. But when I entered my correct password to log in, I only got the error “Failed to start session” in red text.

The article ‘Fix “Failed To Start Session” At Login In Ubuntu 14.04’ helped with that. At the login screen with the error message, I pressed Ctrl+Alt+F1 again to show a terminal, and logged in. Then I ran sudo apt-get install ubuntu-desktop, which installed ubuntu-desktop for the first time. Finally, I ran sudo reboot to reboot.

When the computer finished rebooting, I was able to successfully log in at the login screen and return to using the computer normally.

I dont know if this will help you but, this happened to me after I moved around harddrives from one computer to another. I had a black screen with a comand cursor blinking and taunting me everytime I tried to boot. After reformating and installing and reinstalling. I checked my setup and my comp was booting the wrong hard drive. I swapped the drive order and the bootloader came up.

Again, this was just my experience with the black screen cursur blinking. Solution was:

F2 > boot > swap boot order of drives.

I was unable to switch to other TTYs as is recommended in other answers. I had a blinking cursor, but otherwise my system was unresponsive.

Root cause for me: Bad package upgrade (?improper shutdown?) left my system/dpkg in a broken state.

Detection: After using recovery mode to get to the root shell, I tried apt-get purge lightdm as per previous answers. This command failed, with a message: dpkg was interrupted, you must manually run 'sudo dpkg --configure -a' to correct the problem.

Fix:

1. Get to a root shell if you haven't already. You can do this by booting into 'recovery mode' from grub, and using the keyboard to select the root prompt option.
2. Run dpkg --configure -a.
3. If this was indeed your problem, it will resume some package installation that was not completed. For me, that was a kernel upgrade with an nvidia driver component.
4. Be patient. After it is completed, try reboot and see if it has fixed your problem.