ubuntu 14.04.4 with kernel 4.2: black screen instead of the login screen after installing nvidia driver

I am new to ubuntu and I want to install cuda 7.5 on my computer, but I met problems when I tried to install nvidia driver. I have tried many times and many methods, all failed.
For example, I tried:

sudo apt-get purge nvidia*
sudo add-apt-repository ppa:graphics-drivers/ppa

sudo apt-get update
sudo apt-get install nvidia-361

But I got a black screen instead of the login screen. I have tried all possible drivers, but none worked. The nvidia driver on my dual graphics laptop is GeForce 840M, and my system is:

• ubuntu 14.04.4 with linux kernel 4.2.0-30-generic.

To get into system, I have to run sudo prime-select intel, then I run the following commands after reboot:

When I run lspci -knn | grep -EA2 'VGA|3D'in termal, the information is:

00:02.0 VGA compatible controller [0300]: Intel Corporation 4th Gen Core Processor Integrated Graphics Controller [8086:0416] (rev 06)
  Subsystem: Lenovo Device [17aa:5040]
  Kernel driver in use: i915

01:00.0 3D controller [0302]: NVIDIA Corporation GM108M [GeForce 840M [10de:1341] (rev ff)
03:00.0 Unassigned class [ff00]: Realtek Semiconductor Co., Ltd. RTS5227 PCI Express Card Reader [10ec:5227] (rev 01)

  Subsystem: Lenovo Device [17aa:5028]

I had the same issue.
Create file /usr/share/X11/xorg.conf.d/20-intel.conf with next content:

Section "Device"
 Identifier "Card0"
 Driver "Intel"
 Option "AccelMethod" "uxa"

EndSection

This solution helped me and I hope this will help you