Hi!
Since you are saying that the device fell, it seems like the lower screen may have taken some damage, its connection to the main board may have been disrupted or the ribbon cable may have broken due to the force of the fall. You should try to take it apart and reconnect the lower screen to the main board and visually inspect the screen's ribbon cables. If they show no sign of damage, yet the screen won't work even after reassembly, you should replace it (you can find replacement parts on Aliexpress, eBay or even here, on iFixit).
I will attach a repair guide here to help you: [guide|3750|DSi Lower LCD Replacement]