I updated my 14.05 with Vivid 15.04 HWE stack yesterday and now it does not work. I tried thing that worked for me in the past and that was uninstalling the nvidia driver and installing it again. Then it magically worked once again (I suspect it was DKMS module).
Now I cannot load Xorg. It just does not load and the system loading is stoppend on this step. I tried reinstalling as I mentiond and also installing the newest drivers from ppa:graphics-drivers/ppa and these are showing the same result. With removal of xorg.conf. I also tried to purge nvidia drivers and revert to nouveau, but this does not work so. Surprisingly if I unmount my Nvidia driver from PCI slot and plug HDMI for Intel GPU the system works.
The best possible solution would be how to revert back to nouveau driver. Thanks in forward.
purge
the drivers ? To remove them completely runsudo apt-get purge nvidia*
, perhaps some part was left over if you used something likesudo apt-get purge nvidia-352
???nvidia-common
is depreciated.. New version issudo apt-get install nvidia-352
sudo apt-get purge nvidia-352 && reboot
to make sure it is gone, thensudo apt-get install nvidia 340