Googlade:
"If your Mac has made it to the blue screen, we can rule out some possible problems right off the bat. To get to the blue screen, your Mac has to power up, run its basic self-test, check to ensure that the expected startup drive is available, and then start to load data from the startup drive. This is where it got stuck, which means your Mac is in pretty good shape overall, but your startup drive may have some problems."