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.
I've got an Optimus nvidia Asus laptop. After install Kubuntu 15.04 (I did it in early state of Beta2) I wasn't able to make it work in any other way else than nouveau mode. This is what I did:

-Select nouveau driver in Drivers configuration

-Reboot

-In Terminal type: sudo remove nvidia\* (to delete all files of my nvidia-340 propietary drivers)

-Reboot

-In my Account configuration I selected "auto login" and in SDDM advanced configuration I did it also (this is because the SDDM login screen crashes the startup of desktop with nvidia propietary drivers if it is not in optimus intel mode)

-In Terminal type: sudo install nvidia-346 nvidia-prime nvidia-settings

-Reboot

-After that I had my laptop working in nvidia-mode perfectly.

I also had an issue with "sudo dolphin" that crash my desktop and make the blank screen appeared with nothing else than the mouse pointer. To solve it:

-In the blank screen press Ctrl-Alt-F1 that leads to console

-Type sudo chown username:username ~/.cache/\*

-Reboot

This is another known issue due to change the propietary user to sudo from your user when you use a gui application with the sudo command.

Hope it helps