Chosen Solution
its not water damaged or anything but the phone is clearly turned on but the screen is black
Have you tried putting it into dfu mode, and restoring it? iPhone 4 / 4S / 5 / 5S / 5c / 6 / 6S - how to flash from DFU mode. Password recovery.
Possibly a broken LCD screen , are there any cracks if the LCD has crack this can cause LCD to fail causing black screen , screen replacement could be required to fix your phone