Ubuntu 18.04 failed upgrade from 16.04 LTS

This worked for me:

Open a terminal with: CTRLALTF1 (enter your user and password) then

sudo apt install aptitude

next step

sudo aptitude install ubuntu-desktop gdm3

you will be offered options how to fix the incompatibilities, hit a few times N and ENTER until you are suggested with some removal of old packages and installing of updated one.

then give Y and ENTER to a solution that removes old incompatible packages and installs the new ones.

last

sudo reboot

The above worked for me.

You seem to have got a number of broken packages, as per the error message E: Error, pkgProblemResolver::Resolve generated breaks, this may be caused by held packages.. To resolve, look at the file /var/log/dist-upgrade/apt.log. Locate the packages that are broken and remove them with sudo apt-get remove broken-package. If there are dependency errors, then try to reinstall the package: sudo apt-get install --reinstall broken-package.

Now try fixing dependency errors:

sudo apt-get -f install

Reboot and try upgrading again.

Open a terminal using Ctrl+Alt+T and type:

update-manager -cd

From here, try to update. If there is an error with an option that says partial upgrade, click it. This will restore the GUI. Reboot, and run the command again. There should be no error, and you can proceed with the upgrade via the GUI.