black screen after screen replace

The other few things to check would be the 2 connectors that control the display and the backlight. Those connectors can bend quite easy, verify that they are flat before placing them on the logic board and work from one side to the other to help ensure correct seating. You mention the screen flickered or lit up? Apple can be funny with diagnostic codes. Sometimes the code is displayed via a certain color millisecond splash screen "flicker". Also, the proximity sensor and ambient light sensor can be damaged easily in those repairs and may keep the device from booting to protect itself. Occasionally by disconnecting just the flex cable from the logic board that contains those sensors and the front facing camera, will allow it to boot fully and confirm the issue. Best of luck