Half of touchscreen is unresponsive. How can I make Android only use the top half (rotated by 90 degrees)
Perhaps you could try out something like Assistant menu's cursor facility,


  Unfortunately, I am not sure where you could get one though.


It's you phone's hardware that is not working right? If so, you could just make all touch events in upper half with such a software and emulate that touch occurred in lower half somewhere? You would (most likely) need a rooted phone for installation of such a software though.

The one with Samsung's phones can be accessed from settings/accessibility/dexterity and interaction/assistant menu , I am not sure whether stock android has such a facility. You can then use the lower half screen by scrolling the cursor down, A bit cumbersome , it would be for sure though. You should search for such an app. Which (most likely) be a system app as it has to be available all the time. 

Hope this proves to be helpful.