Hi Zaaied,
Although it's difficult to say for sure, it really sounds as though the proximity sensor on your phone has failed. That sensor is responsible for turning off the screen when you put the phone up to your ear, but the thing is it shouldn't be active unless you're on a phone call. Have you restarted the phone since it went black? The bad thing about it if it is the proximity sensor is that it's built into the motherboard, so it either requires a very skilled repair technician to use microsoldering techniques to replace it, or you'll have to put in a new motherboard. Here's a picture showing the location of the proximity sensor.
That's not to say the proximity sensor is the only possible cause. It could be as simple as a failed OLED display, although this doesn't seem like your normal failure.
I'd try a hard reset but after that I'm not quite sure where to go with it. The screens are far too expensive for me to just suggest replacing it without stronger evidence that it's the problem. This one may be a case where it will be worth your time to take it to a repair shop for a diagnosis of the problem.
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