Why won't my Mac start?
The best bet is to take it to apple if you have it under warranty. They will most likely go in the back and reset it and most likely that will fix it. I had a USB error not that long ago, so I took it to Apple. They reset my computer, and it fixed it. Nothing was deleted/lost, and rebooted perfectly fine. Only problem is it runs a tiny bit slow once you reboot, but it will work itself out after a bit of use.