Yesterday, I’ve upgraded bumblebee on my Asus N73SV, which by itself downloaded and installed a new version of the proprietary Nvidia driver.
During the upgrade process, I was asked to install the maintainer version or my version… and I kept my version.
So after rebooting, I had no Kubuntu splash screen, just a black screen and after a while it just computer just hang with a dead keyboard. So unbootable and only rebootable by pressing the power button several seconds.
After some fiddling around I found out that booting into safe mode, getting into a root shell and starting KDM, everything seemed to work fine.
To keep a long story short, at boot time the X server loaded the Nouveau driver, and it was failing (looping) with some strange IRQ error. So the computer hanged.
The solution: Goto System -> Additional Drivers and enable the NVidia driver.
Reboot, and everything is Ok: Splash screen and Kdm.
A good solution is probably to purge completely the Nouveau driver if you’re really want to use the Nvidia driver. Instructions here: https://help.ubuntu.com/community/BinaryDriverHowto/Nvidia
I reality this is the same issue with the ATI drivers. After upgrading the drivers it is, at least in my case, to explicitly enable them.