On boot tap shift key a few times just after BIOS screen and the GRUB screen will appear ... pick the one saying Ubuntu extra choices or such (not normal,not restore the 3rd one) ... pick root shell option ... Now we will remove all nvidia drivers to revert temporarily back to nouveau drivers
dpkg -l|grep nvidia # to see what you currently have installed
sudo apt-get remove --purge nvidia*
reboot and it should allow you to login normally however you will be using open source nouveau graphics drivers ... I suggest you now upgrade to ubuntu 16.04 ... then to install latest nvidia drivers
System Settings -> Software & Updates -> Additional Drivers
and pick the one saying Nvidia version 367.35 which is refreshingly recent and as a bonus will enable you to install the Vulkan SDK
Above UI listing is also available from command line
ubuntu-drivers devices
ubuntu-drivers devices | grep recommended
If above does not show recent driver version then checkout How do I install the Nvidia drivers?
tl;dr identify if your nvidia card has a companion Intel/AMD graphics card which is used normally such that nvidia is auto selected when needed ... although this was more of an issue in prior ubuntu releases
This command will show your graphics card(s)
lspci -k | grep -EA2 'VGA|3D'
pay particular attention to what is says after
Kernel driver in use: xxxxxxx
Kernel driver in use: nvidia <-- you want this to appear
this repo will make visable recent driver releases
sudo add-apt-repository ppa:graphics-drivers/ppa
sudo apt-get update
sudo apt-get upgrade
from above you typically want to install the most recent driver
sudo apt-get install nvidia-xxxxxx # <-- the latest one
sudo apt-get install nvidia-367