Why is 1/4 of my screen black

It's usually the LED array - I've had it happen on an E6440 and it's usually a replacement LCD :-(. Sometimes you can service the strip but the modern LED panels that they use now which are sealed with tape are all disposable with no serviceability. The bridge screens like some early transition machines where they used a CCFL driver to run the panels can sometimes be fixed (as well as the early post bridge panels), but ANY of the thin ones that came out since ~2014 are disposable. You have to replace the screen as a unit. The HP Spare# is N65679-001. This is a $700 part.

Here is the SM if you want to do the repair: https://h10032.www1.hp.com/ctg/Manual/c0...