Press the F2 key a couple times, making the screen brighter. It could be turned down all the way, seeming non functional.
You can also try connecting the iBook to an external display to determine weather you have a problem with the LCD screen or the video card.
Any problems should warrant you tracking down whoever sold you a lemon and demanding a full refund. Save yourself some grief –- you may end up in an endless spiral of debt trying to fix this thing up.