Hey, Berkeley Mike is on the right track. Describe the symptoms, exactly, and maybe someone can help. Start with the basics, like resetting the PRAM. If that doesn't work then boot from the system CD. Simple OS corruption, along with about 100 other things, can cause a failed boot. I am Apple certified, for whatever that is worth, and have been for many many years. I have been inside everything Apple has produced in the last 20 years. Apple builds fairly robust stuff, and real failures are pretty rare. The problem is almost always user born..nobody like to hear that but it is the truth...and no disrespect intended. Don't fret, I bet it turns out to be nothing much.