ACPI PPC Probe failed. Starting version 219 NVIDIA
I had the same problem. For me, two solutions worked:
Use the X.Org Nouveau display driver (by purging the Nvidia binary drivers as described in other answers). The drawback is that it will impact the graphics performance which is better with an official Nvidia driver.
Install the Nvidia driver and boot with the 'nomodeset' flag. To do this, configure Grub as described in the answer to this question
This will show a really ugly splash screen during booting, but once you passed it you reach sddm and have a nice graphics performance.
From the looks of it, nobody's mentioned how this is filed as a bug?! o.0
Have a look @https://bugs.launchpad.net/ubuntu/+source/systemd/+bug/1432171
I seemed to have fixed it but I still am not very satisfied since I had to uninstall the nvidia-346 drivers. I hope 15.04 will fix this issue. Else all works again. Thanks to Tim for helping me as much as he could.
My Solution:
First I changed the cables from DVI dual-link to DVI. The dvi cable must be connected to the motherboard. Then I could finally see something on boot. If you see a black screen try to press enter since grub might be on. Then you should see a little _ blinking on the top left side of your screen. Once you let it blink for ~ 30 seconds you can press Ctrl+Alt+F1 and enter the terminal (tty). From there login and enter:
sudo apt-get remove --purge nvidia-XXX
Where XXX is comes your driver version.
For instance I had the NVIDIA GeForce 346.59 drivers so I had to enter
sudo apt-get remove --purge nvidia-346
then reboot with the cable connected to the GPU again. It should no longer show the error above and will boot back into Ubuntu.
EDIT:
If you installed the drivers from software & packages and not from a command line there might be a slight difference!
you can always check all nvidia packages by typing:
dpkg -l | grep -i nvidia
you might find something like this:
nvidia-xxx-update
to remove that use the purge command above but instead of nvidia-xxx use the one you can find by using
dpkg -l | grep -i nvidia