I may have a solution, I've been trying to figure out what happened for a few days now.
Deleting all ~/.cache and ~/.config is not a great idea, as those could be needed but. . .
In ~/.cache, I have deleted only the files with a 14.04 boot DVD, but I have kept the folders:
activityswitcher_wallpaper_preview.kcache
event-sound-cache.tdb.093cbe7e51f0423cb8a1384115f450a1.x86_64-pc-linux-gnu
icon-cache.kcache
ksycoca5
ksycoca5stamp
motd.legal-displayed (0 kb)
plasma-svgelements-air_v1.0
plasma-svgelements-breeze-dark_v0.9.7
plasma_theme_breeze-dark_v0.9.7.kcache
[...]
plasma_wallpaper_preview.kcache
After rebooting, the desktop was fine.
Even though you assume that it is not a graphics drivers issue ... trying this solution might be a chance to get your problem solved - it worked in many similar cases ... first remove the drivers.
Highlight the Ubuntu entry in the GRUB boot menu and press the E key.
Add nouveau.modeset=0
to the end of the linux line - press F10 to boot.
On the login screen press Ctrl+Alt+F1
enter user name and password ... execute :
sudo apt-get purge nvidia*
sudo reboot
Identify the graphics adapter ... execute :
lspci -k | grep -EA2 'VGA|3D'
For GEFORCE 400 - 700 I recommend to install the NVIDIA drivers version 352.
For GEFORCE 800 - 900 I recommend to install the NVIDIA drivers version 358.
Installation instructions for NVIDIA drivers version 352 :
Highlight the Ubuntu entry in the GRUB boot menu and press the E key.
Add nouveau.modeset=0
to the end of the linux line - press F10 to boot.
On the login screen press Ctrl+Alt+F1
enter user name and password ... execute :
sudo apt-get update
sudo apt-get install nvidia-352
sudo reboot
Installation instructions for NVIDIA drivers version 358 :
Highlight the Ubuntu entry in the GRUB boot menu and press the E key.
Add nouveau.modeset=0
to the end of the linux line - press F10 to boot.
On the login screen press Ctrl+Alt+F1
enter user name and password ... execute :
sudo add-apt-repository ppa:graphics-drivers/ppa
sudo apt-get update
sudo apt-get install nvidia-358
sudo reboot
Note :
Generally you should use the NVIDIA drivers from the Ubuntu repositories. But when there are problems with the drivers ... then you can install the latest official NVIDIA drivers from the GPU Drivers PPA. This often leads to properly working graphics, especially on new NVIDIA adapters.
Best Answer
I had the same problem, Logging in the terminal mode (Alt+F1) and running the command:
and rebooting fixed my problem.