These machines have had problems with the graphics cards in them failing but that normally shows glitches on the display. If you hold the alt-key while booting up, you should see a recovery partition. If you boot from that, it will let you run Disk Utility to check the drive for any problems. You can also boot up holding the shift-key to get to safe mode, which will let you backup data if it's not the drive failing.
8/28/14 at 8:23am