MY i5 MM(Late 2012), 2.5GHz,16GB Ram, 500GB HD will not boot. It does turn-on as I am able to hear the power button sound when pressed, however, all I am only able to see the Apple White Logo under which is the white progress line with no advancement. . I am unable to access the log-in screen in which I enter my password. Last month I upgraded the OS X from "Mountain Lion" to "El Capitan" in which my MM was running ok until today. Any thoughts on this matter will be greatly appreciated.