Why is my mac just blowing air instead of starting up?

I have a 2003 ibook G4 that's been working fine for the last four years or so. Until yesterday. When I press the power button now, nothing comes up on the screen (it remains completely black as if it were shut off). There is no "i've just turned on" chime either. All that happens is that the fan (i guess) starts blowing like crazy - this whirring, buzzing, blowing sound with air blowing out of the back. If I leave it like this it will simply continue indefinitely. It's really quite loud and frightening. If i try to shut it off by holding the power button down nothing happens either. To stop it I have to unplug the cord and pop the battery. Does anyone have any idea what's going on? Is my computer finally just dead?