How can I uninstall a nvidia driver completely ?

For Ubuntu 12.04

Commands can be executed to terminal. You can open a terminal with Ctrl + Alt+T keys combo.

If you remove --purge the nvidia driver you will be OK. No need to blacklist something, but sometimes maybe a force-load of the nouveau module needed.

First uninstall completely the driver.

Search what packages from nvidia you have installed.

dpkg -l | grep -i nvidia

except the package nvidia-common all other packages should be purged.

If you want to be sure that you will purge everything related to nvidia you can give this command

sudo apt-get remove --purge '^nvidia-.*'

the .* in the end means (Purge everything that begins (^) with the name nvidia-)

BUT

above command will also remove the nvidia-common package and the nvidia-common package has as a dependency the ubuntu-desktop package.

So after above command you should also give the installation command for ubuntu-desktop package

sudo apt-get install ubuntu-desktop

Also sometimes the nouveau driver get blacklisted from nvidia driver. With purge command it should UN-blacklisted. If you want to be sure that nouveau will be load in boot, you can force-load it by add it to /etc/modules

echo 'nouveau' | sudo tee -a /etc/modules

Last , search for the xorg.conf file and remove it as well

sudo rm /etc/X11/xorg.conf

In summary

sudo apt-get remove --purge '^nvidia-.*'
sudo apt-get install ubuntu-desktop
sudo rm /etc/X11/xorg.conf
echo 'nouveau' | sudo tee -a /etc/modules

Although all above commands not needed, this is my way to completely purge the nvidia driver and use the open source nounveau.

I just used the nvidia-uninstall.

sudo nvidia-uninstall

In my case I got the driver directly from the nvidia website.

I realize that this is an old answer, but I have to add an answer here for sake of clarity and system stability.

First off, the * is an expansion operator for the shell which will grab everything and remove a lot of files you don't need to remove. The safest way to remove the nvidia driver is to do

$ dpkg -l | grep nvidia

Search for nvidia-xxx.xx version or nvidia-driver-xxx.xx version and then type

$ sudo apt purge nvidia-xxx.xx

Or:

$ sudo apt purge nvidia-driver-xxx.xx

It will only remove that package but will also flag its dependencies for removal.

To remove the dependencies is easy.

$ sudo apt autoremove
$ sudo apt autoclean

So for example, if you have the 390.xx package installed, it would be.

$ dpkg -l | grep -i nvidia
ii  libnvidia-cfg1-390:amd64                   390.48-0ubuntu3                     amd64        NVIDIA binary OpenGL/GLX configuration library
ii  libnvidia-common-390                       390.48-0ubuntu3                     all          Shared files used by the NVIDIA libraries
ii  libnvidia-compute-390:amd64                390.48-0ubuntu3                     amd64        NVIDIA libcompute package
ii  libnvidia-compute-390:i386                 390.48-0ubuntu3                     i386         NVIDIA libcompute package
ii  libnvidia-decode-390:amd64                 390.48-0ubuntu3                     amd64        NVIDIA Video Decoding runtime libraries
ii  libnvidia-decode-390:i386                  390.48-0ubuntu3                     i386         NVIDIA Video Decoding runtime libraries
ii  libnvidia-encode-390:amd64                 390.48-0ubuntu3                     amd64        NVENC Video Encoding runtime library
ii  libnvidia-encode-390:i386                  390.48-0ubuntu3                     i386         NVENC Video Encoding runtime library
ii  libnvidia-fbc1-390:amd64                   390.48-0ubuntu3                     amd64        NVIDIA OpenGL-based Framebuffer Capture runtime library
ii  libnvidia-fbc1-390:i386                    390.48-0ubuntu3                     i386         NVIDIA OpenGL-based Framebuffer Capture runtime library
ii  libnvidia-gl-390:amd64                     390.48-0ubuntu3                     amd64        NVIDIA OpenGL/GLX/EGL/GLES GLVND libraries and Vulkan ICD
ii  libnvidia-gl-390:i386                      390.48-0ubuntu3                     i386         NVIDIA OpenGL/GLX/EGL/GLES GLVND libraries and Vulkan ICD
ii  libnvidia-ifr1-390:amd64                   390.48-0ubuntu3                     amd64        NVIDIA OpenGL-based Inband Frame Readback runtime library
ii  libnvidia-ifr1-390:i386                    390.48-0ubuntu3                     i386         NVIDIA OpenGL-based Inband Frame Readback runtime library
ii  nvidia-compute-utils-390                   390.48-0ubuntu3                     amd64        NVIDIA compute utilities
ii  nvidia-dkms-390                            390.48-0ubuntu3                     amd64        NVIDIA DKMS package
ii  nvidia-driver-390                          390.48-0ubuntu3                     amd64        NVIDIA driver metapackage
ii  nvidia-kernel-common-390                   390.48-0ubuntu3                     amd64        Shared files used with the kernel module
ii  nvidia-kernel-source-390                   390.48-0ubuntu3                     amd64        NVIDIA kernel source package
ii  nvidia-prime                               0.8.8                               all          Tools to enable NVIDIA's Prime
ii  nvidia-settings                            390.42-0ubuntu1                     amd64        Tool for configuring the NVIDIA graphics driver
ii  nvidia-utils-390                           390.48-0ubuntu3                     amd64        NVIDIA driver support binaries
ii  xserver-xorg-video-nvidia-390              390.48-0ubuntu3                     amd64        NVIDIA binary Xorg driver

To verify, you can do

$ apt-cache search nvidia | grep driver
nvidia-settings - Tool for configuring the NVIDIA graphics driver
ubuntu-drivers-common - Detect and install additional Ubuntu driver packages
vdpau-driver-all - Video Decode and Presentation API for Unix (driver metapackage)
xserver-xorg-video-nouveau - X.Org X server -- Nouveau display driver
nvidia-340-dev - NVIDIA binary Xorg driver development files
nvidia-384 - Transitional package for nvidia-driver-390
nvidia-384-dev - Transitional package for nvidia-driver-390
nvidia-driver-390 - NVIDIA driver metapackage
nvidia-utils-390 - NVIDIA driver support binaries
xserver-xorg-video-nvidia-390 - NVIDIA binary Xorg driver
bumblebee-nvidia - NVIDIA Optimus support using the proprietary NVIDIA driver
kubuntu-driver-manager - Driver Manager for Kubuntu
kubuntu-driver-manager-dbg - Driver Manager for Kubuntu -- debug symbols
nvidia-common - transitional package for ubuntu-drivers-common
nvidia-304 - NVIDIA legacy binary driver - version 304.137
nvidia-304-dev - NVIDIA binary Xorg driver development files
nvidia-340 - NVIDIA binary driver - version 340.107
nvidia-387-dev - Transitional package for nvidia-driver-390
nvidia-387 - Transitional package for nvidia-driver-390
nvidia-390-dev - Transitional package for nvidia-driver-390
nvidia-390 - Transitional package for nvidia-driver-390
nvidia-driver-396 - NVIDIA driver metapackage
nvidia-utils-396 - NVIDIA driver support binaries
xserver-xorg-video-nvidia-396 - NVIDIA binary Xorg driver

Once you've targeted the package to remove, do

$ sudo apt purge nvidia-390 (nvidia-driver-XXX) -y
$ sudo apt autoremove -y
$ sudo apt autoclean

Make sure to install whatever driver you plan on using right after you do this and if you don't have livepatch because of whatever reason, just reboot your system and you should be good to go.

This way you don't ever have to worry about removing system dependencies while clearing out your drivers. You'll have a sane and stable system afterwards and don't have to worry about re-installing other packages that your system may depend on. That's apts job, not yours.