Hi @sumzzz,
When you say “…..or take a photo a small part of the screen is black.” do the photos show a complete picture when downloaded and viewed on a computer?
If the downloaded images are OK then most probably there is a problem with the camera’s LCD screen. Either it is faulty or it has a loose flex cable connection due to the fall.
Here’s a [https://www.youtube.com/watch?v=2EMHoY-JkwA|video] that shows how to replace the LCD screen in your camera. This should allow you to check the cable connection first and if it appears to be OK then to replace the [https://www.ebay.com/itm/NEW-LCD-Display-Screen-for-Nikon-D3400-Digital-Camera-Repair-Part-/251704339624|LCD screen] (supplier example only to show what it looks like and the cost - there may be other suppliers that suit you better. Just search for ''Nikon D3400 LCD screen'' to get results).