After upgrading the graphics card, the system does not start properly. It boots to black screen without showing up the user interface. Please help me.
I had the same problem after having installed bad drivers for my graphic card, so in my opinion it's a problem related to some not working open source driver that are being installed by elementary. You can try downloading the correct drivers from your graphic card's producer's website and install them manually from the command line interface, this should solve the problem. In my case I had to reinstall the system, if you have nothing to lose, also try to install elementary again.