Try reseating the cables for the top screen. (for bottom screen too if you had also disconnected that at some point) They are super finicky and if they aren't perfectly inserted you will have issues turning the console on.
If all else fails and it still fails to turn on, plug in the old top screen again. If the console powers up properly again, then you maybe got a defective top screen? Assuming your console was still powering on with the previous broken top screen you were replacing. You didn't mention why you were replacing the top screen. would assume it's cracked or something?