Do You See A Folder With a ? While Starting Up Your Mac?
If you try to start up your Mac, or MacBook and all you see is a folder on the desktop with a question mark, or a flashing globe, that means your machine is not booting to the hard drive. It can't find the system software. In some cases this can be caused by a defective disc in the optical drive, or even a faulty usb drive, but in most cases this is caused by a bad hard drive, or the MacOS is corrupt.
If you have this problem, and you can boot to some other media like a MacOS dvd or some other type of disc, you may try to run utilities and determine if the hard drive is failing. If the hard drive tests good the problem may be that the operating system is just corrupt.
In some cases the booting problem can even be fixed without a user losing all of their data, but not always. It all depends on how bad the operating system is corrupted, or if the hard drive is going bad.
If your MacBook will boot normally after showing a question mark or a flashing globe for just a few moments, then you may need to simply reselect the primary startup disk in Startup Disk preferences.