Note - This message only aims at proprietary nvidia drivers users. I'm not sure if your laptops has one, but it may help for other people that were in the same situation. :
I just encountered a similar issue after updating my Kubuntu 16.04 setup (it already worked fine with it). Messing around KDE packages didn't help. However I noticed that Plasma was actually working but I didn't get any real visual feedback (right click worked as if plasma was properly started). I could start in a konsole using alt+f2 and blindly launching it. Manually restarting KWin (
kwin_x11 --replace) showed me that compositing got disabled because of the lack of GLX extension in the X server. It was confirmed by glxgears not willing to start.
So I dropped in a TTY, logged in,
sudo service lightdm stop (note : the default DM under Kubuntu is sddm so it most likely would be
sudo service sddm stop), reinstalled the nvidia drivers, rebooted and it was back working.
I wonder if it was just about the X.org config (so try messing around
nvidia-xconfig first) that got replaced or if the upgrade nuked up some GLX libraries.
Hopefully this will help you and some others going by!