if you had used “apt-get update”….
the new update contains some bugs for nvidia drivers….
To slove:
1.get the nvidia software

sudo dpkg -l |grep nvidia

rc  nvidia-304                               304.132-0ubuntu0.16.04.2                      amd64        NVIDIA legacy binary driver - version 304.132
rc  nvidia-opencl-icd-304                      304.132-0ubuntu0.16.04.2                      amd64        NVIDIA OpenCL ICD
rc  nvidia-settings                            361.42-0ubuntu1                               amd64        Tool for configuring the NVIDIA graphics driver

2.remove nvidia all

sudo apt-get remove nvidia-opencl-icd-304 nvidia-304 nvidia-settings

3.restart your computer

init 6