"Failed to start session" after interrupted upgrade to 14.04

While upgrading to 14.04 using sudo do-release-upgrade my network connection was interrupted and several packages failed to upgrade.

Running do-release-upgrade again reported No new releases found but apt-get listed many packages "kept back", so it seemed the system was in an intermediate state. I ran apt-get dist-upgrade which installed those packages again, then apt-get autoremove after which apt seemed happy.

However, after restarting I cannot log in at the GUI prompt. After entering my password the error "Failed to start session" appears in red. I am still able to log in from the command prompt, but I don't know what to do to solve or debug the GUI log in failure.

Solution 1:

After issuing the following command I was able to log in again:

sudo apt-get install ubuntu-desktop

I had previously removed update-notifier which caused some other packages to be removed, some of which it seems were necessary to successfully perform the distribution upgrade.

Solution 2:

For me the answer was in my /etc/lightdm/lightdm.conf - I'd been running my 12.04 LTS in ubuntu-2d mode, had to revert back to just ubuntu and that fixed the issue