It is usually a failure of the solder joints beneath the graphics chip. If you press down hard on the left of the trackpad you may get a display, as it puts the chip back in contact with the logic board. There was a number of recalls to replace the faulty boards and a class action against Apple in the Netherlands. You can sometimes fix it by taping a 50p piece to the heatshield below the graphics chip, so it presses on it and keeps it in contact, but it means taking the back off the iBook and this is not for the faint hearted. There's tons of stuff on Google about this. If the earlier suggestion of using an external monitor works, it's a broken cable to the LCD screen, where it passes through the hinge, and not the chip coming away from the board.