Forcing Your Mac to Boot Correctly
Different methods can be used to resolve this issue, all of which involve forcing your Mac to boot in a non-standard way; various power-on commands are available for Macs running OS X.
Begin by inserting the Apple hardware test disk and holding X as you boot your computer, which will force a check of your hardware. Any problems will be listed here – note that if this is bad news, your Mac may need replacing.
Various keyboard shortcuts are included in Mac OS X to allow you to boot your computer with additional or few functions. For instance, you can hold down the SHIFT key while pressing the power button to start your Mac in safe mode, and in some cases this might work.
You might also try resetting the PRAM, the temporary memory area that stores various settings for your Mac. Resetting is done by first ensuring that your Mac is powered off (perhaps removing the power cable for a few seconds) and then, with the APPLE, OPTION, P and R keys all depressed simultaneously (small-handed people may need assistance at this stage) press and hold the power button to start your Mac.
Another trick with the power cable is to reset the SMU/PMU, which can be done by disconnecting your Mac from the mains for two or so minutes. Booting your Mac after this might prove successful, but if not, don’t worry, as there is another solution.