You should be given an unix-style login prompt, so enter your username and password there. This might be incompatible with your graphics card therefore, search with sudo apt-cache search nvidia-+$ for the package you need, and install it.

For example, on Ubuntu 16.04, nvidia-current points to the version: 304.131-0ubuntu3. In that case, search the latest compatible driver for your video card and install it. Please note that sometimes nvidia-current might install the wrong driver. If you're lucky enough, problem solved, you should be able to login to Unity. TL DR, just try logging into the shell ( Ctrl+ Alt+ F2 or whatever F between F1 and F6) and type sudo add-apt-repository ppa:graphics-drivers/ppa Its installer is quite capable too and it uninstalled successfully my hack-installed unstable version from Nvidia website.

The solution here was to install package nvidia-current from Ubuntu repos it is an awfully outdated version, but it's tested properly at least. I realized I have Nvidia drivers from official website which are not really stable and tested (so I've read and also experienced in the past). I had this and after looking at /var/log/Xorg.0.log I found out that it's a Nvidia problem (there was a line saying Xlib: extension 'GLX' missing on display ':0).

Parrot security os login loop