If your graphics driver is not getting updated with your kernel, it will stop working after a kernel upgrade and your system will revert to the nouveau or frame buffer drivers to run your video hardware.
This happened to me when I installed drivers by downloading them from the NVIDIA website. To solve the problem I added the Graphics Drivers PPA and installed the NVIDIA drivers through ubuntu-drivers command.
Start by uninstalling your old NVIDIA driver,
sudo apt-get purge nvidia-*
and then follow these steps:
1. Add the PPA
sudo add-apt-repository ppa:graphics-drivers/ppa
sudo apt-get update
2. Install Video Drivers For Your Card
Now as posted in this answer, you need to install the recommended driver for your card.
sudo ubuntu-drivers devices
This will output something similar to this:
== /sys/devices/pci0000:00/0000:00:01.0/0000:01:00.0 ==
vendor : NVIDIA Corporation
modalias : pci:v000010DEd00000DDAsv000017AAsd000021D1bc03sc00i00
model : GF106GLM [Quadro 2000M]
driver : xserver-xorg-video-nouveau - distro free builtin
driver : nvidia-304-updates - distro non-free
driver : nvidia-304 - distro non-free
driver : nvidia-331 - distro non-free **recommended**
driver : nvidia-331-updates - distro non-free
Then just install the driver labeled "recommended", so in this example it would be:
sudo apt-get install nvidia-331
Once this is done reboot and you should be good to go. Next time your kernel gets updated, your video driver will get updated with it.