Why is there a black screen now?

Prior to the last question I made, I did, in fact checked everything that's wrong but one which is the AC adapter that died and not the laptop at all. All I need is to use a spare AC adapter that I had, plugged it in and it actually works. However, there is seemed to be a problem. My laptop detected that the LCD cable wasn't installed in the diagnostics. I wasn't so sure if it's because I messed up when I was trying to see if there's something wrong inside the laptop or it's something about the AC adapter that I'm currently using. Either way, when I closed the diagnostics, the screen turned black and all I see is a miniscule white block that keeps flashing every second and I can't even do anything about it. How do I even fix this mess??