HI guys,
Thanks a lot for your help. Yes, indeed I recently added another GPU.  I will reinstall Nvidia driver now. But could I ask how to do this in Ubuntu? The problem is I also have Cuda 8.0 installed, will reinstalling Nvidia driver makes me reinstall Cuda 8.0?  Please see the pic below to see Nvidia packages in my machine.

Screenshot from 2017-08-22 09-24-58.png1483×530 115 KB

I have searched around, but the answers I found vary. Some say I should use this command,
`sudo apt-get remove --purge nvidia*`

Some says I should use,
sudo nvidia-uninstall

To reinstall, I just use,
sudo add-apt-repository ppa:graphics-drivers/ppa

Is this the correct way? I guess my Cuda 8.0 will not be removed during the process?
Sorry I am relatively new to ubuntu and want confirm before I do this.
Cheers