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.