If you found this page after a Google search, I have to assume you've pretty much done the same thing and got into your browser after booting up in low graphics mode or used another PC to search for a fix. I use one of two options at this point, depending on how much of a hurry I'm in to get my graphics working again and I've documented both below hoping it'll be of some help to other casual Linux users, who just want to get back to their movie, game or web page as quickly as possible. Mind you, I'm not an authority on Linux administration - both approaches below just happen to 'work on my machine'!
Revert to booting your older kernel version
This is the easy way out; assuming your using grub, simply navigate to /boot/grub, edit menu.lst as root and change the 'default' setting from 0 (the most current kernel) to 2 (the one that was working perfectly well until you upgraded). Save and reboot, and you're fine until you run an update that upgrades your kernel again, at which point you have to change the default to 4 and so on.
Updating to the latest NVidia Drivers
This is obviously the better solution, but takes a little longer.
a) First, you need to go to the nvidia download page and get the right driver for your graphics card.
b) Next you have to restart your system and get to a command prompt using the boot options, because if you're in some restricted graphics mode or a command prompt you managed to get to after having tried to boot normally, you have a broken X windows running somewhere in there and NVidia refuses to upgrade the driver while it is. I can never stop X to it's satisfaction, so I reboot.
c) Run the installer by typing 'sh ./NVIDIA-Linux-x86-[driver version]-pkg1.run' as root. After getting you to agree to some stuff, it'll tell you the old driver will be removed, try to find some relevant binaries, typically fail to do that and compile from source, then install itself, ask your permission to update the x-config and will finally tell you it's done. Reboot at this point and you should be fine.
I've been doing this for some time, so can't remember if I had to set-up any other pre-requisites the first time I upgraded my drivers. Please do comment if I've missed something.
