I recently updated a machine from ubuntu 10.04 to ubuntu 10.10. After reboot the nvidia driver wouldn't load and /var/log/Xorg.0.log contains:
[ 25.820] (EE) NVIDIA: Failed to load the NVIDIA kernel module. Please check your [ 25.820] (EE) NVIDIA: system's kernel log for additional error messages. [ 25.820] (II) UnloadModule: "nvidia" [ 25.820] (II) Unloading /usr/lib/xorg/extra-modules/nvidia_drv.so [ 25.820] (EE) Failed to load module "nvidia" (module-specific error, 0) [ 25.820] (EE) No drivers available.
The solution was to run
$ sudo dpkg-reconfigure nvidia-current
which caused nvidia modules to be rebuilt. This showed me I was missing source for the running kernel version, so I then installed nvidia-185-kernel-source which was enough to get it back on its feet.