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On some rare occasions, after an upgrade; due to compatibility issues between hardware/software components,  when trying to logging you might only see a nasty black screen, or the OS will bring you back to the logins screen; meaning that you are not able to log in at all into Ubuntu.

In most cases, the solution requires direct access to shell and manually make some adjustments. 

Open shell view

You can boot on shell root (no need to login) or press CTRL+ALT+F5 to switch to shell view.

Login and initial prompt

Create a test account for discarding configuration issues.

Creating a temporary account will help to discard any configuration issue in the home directory. The following commands will create a new temporary account for testing purposes:

sudo adduser temporary

sudo systemctl reboot

If you are not able to log in with the new temporary account it means that the issue is within your original account’s home folder; so for this reason, the old configuration files need to be deleted in order to allow a normal login:

rm -rf  ~/.config

rm -rf ~/.local

rm -rf ~/.cache

rm -rf ~/.nvidia-settings-rc

rm ~/.nv

sudo systemctl reboot

If after rebooting the issue persists, please go back to shell view and continue troubleshooting.

Reinstall the login manager

After an upgrade, some core UI related components might be causing a conflict, so it is recommended to reinstall the GNOME Display Manager and the desktop environment by running the following commands:

sudo apt install –reinstall gdm3 ubuntu-desktop gnome-shell

sudo systemctl reboot

If after rebooting the issue persists, please go back to shell view and continue troubleshooting.

Reinstall NVIDIA Driver

A recent NVIDIA update might also be causing the login issues, so for this reason it would need to be reinstalled. To remove the NVIDIA driver, run the following commands:

sudo apt purge ~nnvidia

sudo apt autoremove

sudo apt clean

Removing NVIDIA

After the NVIDIA driver has been removed, reinstall it by executing the following commands:

sudo apt update

sudo apt full-upgrade

sudo ubuntu-drivers autoinstall

sudo systemctl reboot

In most cases, after following these steps, Ubuntu should be able to normally log into Ubuntu; if the problem persists you might need to consider to re-install your OS or look more deeply into any hardware issue.