Hi
This is usually one of two things
1)a dodgy screen connection,the screen connector on this calculator is soldered to the main board
this can be sometimes be fixed by resoldering the connections with a soldering iron
Here’s a guide on doing it
[link|https://m.youtube.com/watch?v=mY-tI4WQDfA|https://m.youtube.com/watch?v=mY-tI4WQDfA|new_window=true]
2) it could be that the screen is internally damaged ,if this is the case the screen will need replacing
The guide linked above has the disassembly guide in it
If you need a part you will probably need to get a donor calculator as these parts don’t seem to be readily available
Hopefully this helps
Any questions please ask
Thanks:-)