If this was a full [guide|149125|Front Shell Replacement|new_window=true] then chances are that the cable that connects the screen to the motherboard was not seated correctly. My advice is to go through the [guide|148986|Screen Replacement guide|new_window=true] up to step 26 and make sure that the cable for connecting the screen is properly seated, and test if you can get your screen's backlight to come on. Otherwise, your screen may have become damaged during the shell swap and would need to be replaced.