I have two possibilities of problems.

(Sorry, i can't write english well. I used translation. )

I did not attach the smart cover magnet to the replaced glass during the operation of replacing the glass of the iPad Mini 2, could this cause the iPad's screen not to light up? The current state is a sound, but the sound that is opened and closed continuously runs periodically and the screen does not come out.

If this is not the case, it is likely that the LCD is broken by simply handling a lot of LCD.