How to diagnose my dead screen?

So my late 2015 imac 5k's screen died today after i upgraded the cpu and went to place the screen back on. Everything works otherwise, i just ave to shine a light on it to see(im writing this post on it now). I was just wondering if there is a way to diagnose what caused this? I really dont want to buy a whole new panel, just the faulty part, or if the screen is fine but something else may be having issues. Thanks