When attempting to upgrade Ubuntu 17.10 to version 18.04 it would give a message about 'failed to update Ubuntu-Desktop.'
Failed to start session after the upgrade and restarting. Issue was dependency that was broken: "ligbm1"
Answer from k.child90 from askubuntu.com below:
https://askubuntu.com/a/1080242/731657
"I took this along to a laptop clinic who solved the issue. The login screen, importantly, looked identical to the 16.04 Unity login screen which betrayed the problem in transferring to 18.04 (which has a very different looking login screen).
In TTY attempting to install certain packages gave unmet dependencies, and following the chain down gave the ligbm1 package at the base. This is the dependency for a lot of unity and xserver packages (attempting startx also failed). Uninstalling ligbm1 and then installing it again allowed installing ubuntu-desktop without error." --k.child90
