Replaced iPhone 6 screen and now the screen goes black and lock buttons unresponsive?

I just had my iPhone 6 screen replaced at a local phone repair shop. The shop is reputable in comparison to other local repair shops and therefore I went with it.

The screen took around 20 minutes to replace and it was fine the first few minutes after. I then locked the phone and went to unlock it, and the screen was blank. The phone repair man fiddled with it for a few minutes and again, it was fine. I went to the car to drive home and again, the screen was black and the lock buttons were unresponsive. By the time I arrived home again, it was working. And then I locked it again and it went blank.

Why is this happening? What can I do to fix it?

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