After months of good use my wife's 12" iBook has an odd behavior. When you hit the power button you get the standard tone and then nothing happens. It sounds like the machine is booting and inserting a CD brings the familiar spinning up sound, but the screen never comes on. I have plugged it into an external monitor and still nothing. Other things I have done are remove the battery and unplug it and repeat. Still nothing. Any suggestions on debugging before we call Apple?