I recently upgraded my Mint 17.3 to Mint 18 using mintupgrade and following the instructions here, and unluckily had a powercut while in the middle of the upgrade. When I rebooted, some things had changed, like the logon screen had new background images, and the grub menu now said ‘Mint 18’, but as soon as I logged on, mdm crashed and there was a dialog saying that XWindows had crashed within seconds of starting. The popup dialog said to check ~/.xsession-errors, which contained this error:
initctl: Unable to connect to Upstart: Failed to connect to socket /com/ubuntu/upstart: Connection refused syndaemon: no process found
A quick Google found this question with the same error message, and following the suggestion to run ‘sudo apt-get install cinnamon’ fixed my issues. I restarted mdm with ‘sudo service mdm restart’, logged on and eveything was good.
I’m not entirely sure how much of the upgrade completed, so re-running ‘mintupgrade upgrade’ again still prompted for a number of packages to be deleted or upgraded. Completed the upgrade, rebooted, and now everything looks good.
This could have been a lot worse, but luckily was able to recover with no noticeable issues so far. And Mint 18 looks great! (I like the new window animations!)