This exact thing happened with me and my iMac G5 recently...tried all of the above and still, couldn't get to 'Loading OSX'...tried booting in 'safe mode' and that did the trick! To boot in safe mode, press the power button...as soon as you hear the Mac tone (noise from speakers when you turn it on) press and hold down the 'shift' key....once you see the 'Loading OSX' screen you can let go of the shift key. Worked fine...rebooted and still working without a problem for me!