If you replaced the display you might have dislodged the magnet held in the lid frame, or the hall sensor is bad
Using a rare earth magnet slide it across the uppercase where the sensor is located you should see the display turn on and off as you pass it across the sensor. Use a piece of tape to mark the location and now using a steel paperclip locate the magnet in the lid and marking its location the two marked locations should meet up. If not you either got the wrong lid assembly or the magnet is missing or got placed in the wrong hollow when you replaced the LCD panel.
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