A few hours ago, I went to go turn on my MacBook Pro (late 2013, retina, 2.4) and it immediately shows the prohibitory sign and the shuts down. I've tried to use Control-Cmd-P-R and holding shift, but none of the keys work besides the power button. I've also tried using an external keyboard and it didn't work either. If any of you know how I can resolve, it would be greatly appreciated if you responded.