Problems upgrading Ubuntu

It wasn't the first time I ran into trouble going from Ubuntu 12.04 to 14.04. I think I even came across a recommendation to not bother making such a jump. But once there's no turning back... Usually the upgrade goes fine — or at least it looks that way. After you shut down, it's a different story. On boot you see only a black screen with a cursor in the top-left corner. Which for an inexperienced user who clicked upgrade in good faith is a shock. The causes can be several. I encountered a few problems myself — one of them was in dpkg, which dpkg-reconfigure sorted out, followed by a broken lightdm and a few other niggles. Which, let's be honest, is not exactly a pleasant surprise. In the end everything got fixed and working... The clear lesson: back up and do a clean install. It takes about the same time and saves you a lot of headaches and potential future problems and complications.