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?

There might be something wrong with the screen that was replaced. Talk to the person who replaced it and ask him what screen he used

THEY REPLACED THE WRONG PARTS, NOTHING WENT WRONG WITH YOUR SCREEN. IT IS QUITE OFTEN HAPPENS TO REPAIR SHOP WHICH WANTS TO MAKE QUICK EASY MONEY FROM CONSUMER LIKE YOU.
TAKE IT BACK, LET THEM REPAIR IT AND REFUND THE SCREEN PARTS MONEY THAT YOU SPENT FROM LAST REPAIR.
REMEMBER, All REPAIR WORK COVERS AT LEAST 30 DAYS WARRANTY OR YOU MAY SUE THEM THROUGH SMALL CLAIM COURT.

Well, your screen maybe not good. And you do not good at the replacing a screen yourself.
SO you should take your phone to a repair store.
You should buy the original screen from the hcqs.cc