Question: I had a windows 7 and Ubuntu 14.10 then i upgraded windows 7 to 8.1 now I cant get Ubuntu to boot

I now have grub but grub will not find Ubuntu

<http://paste.ubuntu.com/11019932>

Answer: 
I think the problem is with the NVidia driver and the use of OpenGL.
When you experience the black screen login on to an other tty and try the following:
- sudo vi ~/.config/kwinrc
- Change Backend=OpenGL to Backend=XRender
- then restart with kill -9 -1

You should now be able to login using Plasma.
It is however very slow but you can sometimes switch back to OpenGL. If you can switch you'll have the performace you expect from Kubuntu.

Before shutting down don't forget to switch back to XRender or you will have to repeat the same steps.
You are probably hitting a kwin bug, I think. Kwin makes too optimistic assumptions on how sync should be performed and this does not work for older nvidia cards.

If this is the case, you can try the following: 

Rather than disabling compositing, try
creating a file `kwin.sh` in `~/.config/plasma-workspace/env/` with the following content

```
#!/bin/sh
export KWIN_EXPLICIT_SYNC=0

```

Then go to the system-settings -> Startup and Shutdown -> Autostart and Check/Add the script as a pre-kde startup file

See also <https://bugs.kde.org/show_bug.cgi?id=348753>