You likely had a partial upgrade happen. The driver needs to be made to
work with the linux kernel so certain kernel modules are needed as well as
the driver. I would suggest that you do either of the following. You can do
an upgrade from the CLI (sudo apt-get upgrade) or you can install another
GUI and work from there. I always have a second GUI just for that reason.
(sudo apt-get install xubuntu-desktop)

If you have broken packages then try: sudo apt-get -f install

If this does not fix it then you can re-boot and use the recovery mode to
fix graphics problems. An alternative would be to try to boot an older
kernel as the driver was already working with that before things went awry.