iPhone 6 Screen is black, phone vibrates and receives notifications

Hi All

So this is an Iphone 6 that a customer brought to me with no physical/water damage on the device/screen. She just said to me that "all of a sudden the screen just went black" I initially thought thet the LCD was broken and tried to install a new screen on the phone but to no avail. I then tried to install 6 other LCDs with the same result, no display. I even tried the customers LCD on another iphone 6 and it worked perfectly fine. Her phone has the OEM LCD and was never opened prior to me opening it up. Also, the phone is recognized by itunes

Here is what else I tried:

*I tried installing 3 new batteries (even though I knew that it wouldnt make a difference since the phone powered on)

• unplugged the battery
• unplugged the battery and LCD and tried holding the power button for 20 seconds to release the static from the phone like others suggested
• Tried soft reset by holding the power/home button for up to a minute

Any other suggestions would be greatly appreciated

Thanks in advance guys