Failed to install CUDA 7.5 in ubuntu 14.04 LTS

Hi,
We have a new assembled ubuntu server, and try to install CUDA 7.5 following the http://docs.nvidia.com/cuda/cuda-installation-guide-linux/index.html#ubuntu-installation (including the pre-installation and post-installation).
But after rebooting, I tried to verify the driver http://docs.nvidia.com/cuda/cuda-installation-guide-linux/index.html#verify-driver

cat /proc/driver/nvidia/version

It shows

cat: /proc/driver/nvidia/version: No such file or directory

And I tried

sudo nvidia-modprobe

The error message is

modprobe: ERROR: ../libkmod/libkmod-module.c:809 kmod_module_insert_module() could not find module by name='nvidia_352'
modprobe: ERROR: could not insert 'nvidia_352': Function not implemented

But

lspci | grep -i nvidia

does find the devices:

04:00.0 VGA compatible controller: NVIDIA Corporation Device 17c2 (rev a1)
04:00.1 Audio device: NVIDIA Corporation Device 0fb0 (rev a1)
05:00.0 VGA compatible controller: NVIDIA Corporation Device 17c2 (rev a1)
05:00.1 Audio device: NVIDIA Corporation Device 0fb0 (rev a1)

The kernel version is

$ uname -r
3.16.0-50-generic

BTW, it’s a headless server, used via SSH.
Any help is highly appreciated!