iPhone screen went black but is on

Hi,

I was using my iphone 5c just a moment ago and was on facebook, then the phone locked up, i couldnt do a thing, then the top quarter of the picture started to do almost like a dissolving thing so that the picture started disappearing, then the screen went blank,

Here's the steps i have taken to resolve this.

Tried turning it off using the power button - Did nothing, screen is still black

Tried holding the power button and home button together - Did nothing, screen is still black

Replaced the screen - Did nothing, screen is still black

Tried plugging it into the computer and doing a reset - Did nothing, screen is still black

However, i can ring the phone and it rings.

So whats the problem? why has my phone randomly died? could it be that the gpu chip has committed suicide?