First thing to do is see if we can verify whether it's a display (LCD) issue or a backlight issue. In a darkened room, hold a flashlight up against the screen, try different angles of both the light and viewing to see if you can make out an image. If you can then it's almost certainly a backlight issue, otherwise it's the whole display.
The most common issue we see with LCD screens that don't work on installation is due to a blown backlight circuit. That usually occurs when the display is being connected or disconnected while the battery is still plugged in. There are pins that are very close together that can short out and destroy the backlight circuits if there's power present when the connectors are misaligned and put in contact. That's why it's so important to disconnect the battery first when disassembling and connected last when reassembling.
Usually it's the backlight on the display that gets blown, but occasionally parts on the motherboard get fried. If you verify that it's a backlight problem, I'd suggest trying another screen; the price for iPhone 6 screens is dirt cheap nowadays so that's what I'd do.