Many customers encounter screens black out after they replace new screen. Most time it is due to the missing or misaligned magnet which is at the left side of front camera, as the photo below.

The common solution is removing the magnet and replace it with customers original magnet, then it will work. Alternatively, a software glitch, a damaged hinge or flex cable, or even a faulty battery could be the root cause.

If the problem still consists, then they may have to seek system configuration from Samsung Service Centre.