Might be a cold solder joint or a broken trace on the LCD board - long ago I had a Powerbook 145 which suddenly developed black jailbars on the bottom half of the screen, I took the LCD apart and with the computer on and displaying an image I started poking at everything on the boards surrounding the display panel, found out when I touched an interconnect said jailbars turned white. I followed that with a multimeter and turns out a trace had gone open circuit - jumped it with a bit of wire and that fixed the screen. 😀