Try this
# sudo apt-get auroremove --purge nvidia-*
# sudo stop lightdm
# sudo apt-get install xserver-xorg-video-nouveau
the above purges your Nvidia and makes the default drivers work
if you want your Nividea to work properly
updating your drivers or removing and reinstalling may work
try below commands
to remove and reinstall
#jockey-text --list to get the list of available options.
#sudo jockey-text -d xorg:nvidia_304 to remove the 304 driver.
#sudo jockey-text -e xorg:nvidia_304 to enable the same driver.
#jockey-text --help
if it says command not found install jockey by (#sudo apt-get install jockey-common)
Then Reinstalling drivers
#sudo ubuntu-drivers devices
#sudo apt-get install nvidia-331
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