I use Debian Jessie, and I am the type who do this command frequently:
sudo apt-get update && sudo apt-get dist-upgrade
Now I am suffering from the consequences. The desktop is completely broken. I googled around, and there was nothing. So I thought something in my laptop is broken at first.
Well, if you are a long enough linux user, this kind of stuff is kind of normal. It always happens once in a blue moon. It happened once when I was upgrading Fedora. It happened once when Debian removed ati driver. This stuff is normal, it's also Debian Testing anyway. One just need to keep calm and fix the problem.
To fix the problem, just go to your terminal, install Gnome, and use it for the time being. If you already have Gnome installed, just switch to use it in your login screen.
Here's the instruction to install gnome, taken from the debian's wiki:
sudo apt-get install aptitude tasksel sudo tasksel install gnome-desktop --new-install
After a few days, I start to saw entries in people's blogs (can't find the link anymore) and in the mailing list.
Basically, the KDE version 4 packages are mixed with version 5. They couldn't upgrade the whole stuff in one go. So until it's over and we get our new shiny KDE 5, we will have to downgrade KDE or just use Gnome (btw, it's also quite shiny).