Unfortunately, these are the signs of a failing LCD display. I had that happen to my old work laptop (HP Compaq nx9010) and there was nothing I could do to fix it. It's not like single dead pixels that have the possibility of being revived....when the lines start appearing, it usually mean the display is on its way to failing. Might take a long time before it kicks the bucket though.
If you're not too afraid of opening up your PowerBook, you might want to check the cables for the video signal to the LCD display and make sure that it hasn't come loose or anything. Ifixit.com has some disassembly instructions that I recommend following if you're going to attempt this. If not, then consider that replacing the display is going to cost just about if not more than the PowerBook itself is worth now, unless you find one on eBay and perform the repair yourself.