Hi,
The LCD screen may have been damaged by the fall. They cannot be repaired, they have to be replaced. As a DIY repair all you can do is open the phone and check that the display flex cable is still connected properly and if so temporarily replace the LCD screen and check if it works OK to check if the problem was the LCD display or perhaps on the systemboard.
Here’s [https://www.youtube.com/watch?v=1lS8YjdkMYca|video] that shows how to replace the screen in your phone.
Replacement screens are available online. Just search for ''Samsung Galaxy J2 LCD screen'' to get results for suppliers.