Most of the time this is simple a corrupt system folder. First try starting in Safe Mode by holding down the Shift key on start up. Next try booting from and external USB source.
Recently I have seen a couple of the SSD blades fail. Remove the blades, then try booting from an external source.
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I forgot, also try booting to the repair partition so you can run Disk Utilities on it. Hold down the option key on boot and select the partition.
Most of the time this is simple a corrupt system folder. First try starting in Safe Mode by holding down the Shift key on start up. Next try booting from and external USB source.
Recently I have seen a couple of the SSD blades fail. Remove the blades, then try booting from an external source.