Can't login after Nvidia driver install v 14.04

I successfully installed drivers to run the Nvidia 750 Ti graphics card in my desktop. Ubuntu ran perfectly for a short time after that. Then, the update manager prompted me to install some updates. I did so and rebooted. After that, when I try to login the screen blinks and goes back to login. I have tried several other guides which solve similar problems but none of them have helped.

I am dual booting Windows 8 and Ubuntu 14.04.

This happens when you install Nvidia drivers using a .run file, downloaded from Nvidia site. This a wrong way to install drivers. After each kernel update you will have to install them again using console, because GUI won't start. Now you can fix it by going to console by pressing Ctrl+Alt+F1. Then login there and run

sudo apt-get purge nvidia*
sudo apt-get install nvidia-331

Then reboot.

If this does not work, then you need to uninstall the driver. You can look HERE for instructions.

Sadly, my experience is that:

sudo apt-get purge nvidia*
sudo apt-get install nvidia-current

is not guaranteed to work.

Sadly it also is true that kernel updates and xorg or other graphics driver updates often break the proprietary driver installation.

I have come to use this scheme:
1. as updates are announced, check for the above type of packages.
2. if none are included in the update, then let them in, no need for more actions.
3. if there are "risky" updates, then:

Press CTRL-ALT-F1, login and then type

sudo -i
stop lightdm
init 3
cd /to/dir/with/NVIDIA.run-file/
bash NVIDIA-CURRENTLY-INSTALLED-VERSION.run --uninstall
apt-get update && apt-get upgrade

if there was a KERNEL update; enter reboot
and restart with the new kernel in recovery mode, then...

cd /to/dir/with/NVIDIA.run-file/
bash NVIDIA-NEW-or-PREVIOUS-INSTALLED-VERSION.run
reboot

I also recommend to update to kernel 3.16, as a means to bypass the numerous/imminent kernel 3.13 updates.
Follow the instructions for Trusty here:
https://wiki.ubuntu.com/Kernel/LTSEnablementStack

NOTE: Avoid later kernels for the time being, i.e. until there is similar support for them.

The cause is when you upgrade something related to .Xauthority with sudo, root will own this file and you cannot login as a user. Just press Ctrl + Alt + F2, login with your username and

sudo chown [your_username] /home/[your_username]/.Xauthority

Nvidia drivers install and register code with dkms. This is not part of the standard install for Ubuntu desktop but if dkms is not installed each kernel update will have the potential to break the Nvidia drivers again.

To keep this from happening, you can install "dkms" yourself:

sudo apt-get install dkms

After installing dkms, reinstall the Nvidia drivers using the run file again. They will be able to register the drivers for recompiling when new kernels are installed.