iPhone 6 screen/blacklight issue

Hello,

I have an iPhone 6, it is a refurb from the Apple Store in Sheffield (Meadowhall) as my previous one had a software issue. I've had it about 9 months.

I dropped it a few months ago and cracked the screen slightly however I've been using it without an issue since, earlier I was chosing a photo to upload to Facebook on the app and my screen went black, I thought the app had crashed so I did a hard reset (home+lock button) and the phone didn't appear to come back on. The phone did come back on and the screen was completely responsive but not on.

I took it outside and noticed that in the sunlight the screen was visible, it is also is visible if you shine a torch on it. I took it to a phone shop to have the screen replaced, they tried 3 screens and none of them fixed the issue. Does anyone know what this could be?