I've finally solved my problem!
For people who have the same problem like I had, here is the solution:

I recommend to make a fresh install of your system, if you already have an issue with drivers.

1. Boot up Ubuntu and make sure you're connected on the internet.


2. Open a terminal with Ctrl + Alt + F1



3. Login and here is what you have to type in terminal:

   
   
   

   sudo apt-get update
   
   sudo apt-get dist-upgrade
   
   sudo apt-get install build-essential
   
   sudo apt-get install linux-source
   
   sudo apt-get install linux-headers-generic
   
   sudo apt-get dist-upgrade
   
   sudo reboot
   
   




4. After reboot, repeat step 2. You'll need here an internet connection, again. Run these commands:

   
   
   

   sudo apt-get install nvidia-current-updates
   
   sudo nvidia-xconfig
   
   sudo reboot
   
   

It is very important to run nvidia-xconfig! If you don't run it, you'll crash your system!

After you've done this, you'll have a working nVidia graphics driver. Cheers!