The problem apparently locked iPhone screen with black screen.
O 6 issue recently seen on iPhone most the new version of iOS 9.
Totally dark screen, without any trace of light or give any sign that it would respond to a command on the buttons. iPhone It seemed totally dark and could not open by pressing the Power / Sleep.
However, the surprise was that sThey could receive calls, the iPhone rang, but it could not be answered, because nothing more than black appeared on the screen.
Causes and iPhone display resolution black / dark
If I described above, with the iPhone, whose screen is not opened at all, we suspect that was a problem because storage space full.
Before you put the iPhone in the box and prepare it for service, it is good to try a "cold" restart. Or forced restart of iPhone. Hard reset.
How to restart an iPhone 6 / 6S - iPhone Hard Reset
Even if nothing appears on the display, We loaded iPhone (To make sure the battery is), we let it load time 5 minutesthen Home and Sleep buttons simultaneously press (side lock / unlock button - right). Keep the two buttons pressed for 15-20 seconds.
Normally, after this time the white or black screen with the logo must appear Apple (apple), and the phone should turn on normally.
After the restart applications installed documenentele, pictures and other data stored on the iPhone, will not be affected.
If this method does not help, try putting the iPhone in DFU mode / Recovery Mode. You need one Windows PC / Mac OS X with iTunes installed, data cable and some attention. this tutorial will not help.