If you still see a faint image of the desktop then try the mayer suggestion. If it doesn't work then:
- the inverter cable might be disconnected from the logic board. I don't think it could be disconnected from the inverter board end cause the connector is firmly inserted in the inverter board.
- shaking the iBook broke the fragile backlight lamp in the LCD panel.
If the backlight lamp is broken you can buy a used iBook G4 LCD panel off ebay and even use a iBook G3 panel that will perfectly fit in your iBook G4