This morning my MacBook Pro with Yosemite had issues starting up. I had run the battery down to 10%, which on mine means it's dead and shuts off hard. This has bitten me before, so I know the trick is to let it charge until at least one light on the battery status lights (on the side of the laptop) is on solid, THEN power it back on. I heard the Wall-E sound, and the screen stayed black. Tried an external VGA dongle, no screen at all. Hard booted it again (power off, power on), got a "Would you like to reset your password?" screen. No, I don't. It had three options, none of which were appropriate. #1 Would only let me enter my password to remove disk encryption (no can do, company owned laptop) #2 and #3 were variants of "my keyboard doesn't work". Click the back arrow appears to hang the app with no selections possible anymore. Hmmm.
Rebooted it hard again, black screen. Again, reset password. Hmmm again.
Ok, clicked back again on the password app, still hung up. Bump the top of the screen and there's an Apple menu! Shutdown or Reboot are the only options (as I recall). Both do what they say, neither fixed this issue.
Get ready for the weirdness.
Reboot to black screen, nothing there. I read something on a site that recommended the following, and it's the craziest Mac workaround I've ever seen.
The black screen is indeed black, BUT the laptop is alive and waiting for your disk decryption password! If you only have one user, you might be able to click dead center and blind-type your password to boot it up. Since mine is corporate-owned, there are a couple of accounts including an admin account, a guest account and my own so it's hard to find where to click.
Cue the weird. Get out your handy phone, and turn on the flashlight app. Shine the flashlight through the Apple on the lid side of the screen. You can now see that little section of the screen using your flashlight as the backlight! The laptop is on, but the backlight is still off! Use the flashlight and use the up/down/left/right/enter keys to get to your user on the selection screen, and then carefully type in your password. Mine chugged for a couple of seconds, realized the backlight was off and turned it back on, and booted up the rest of the way! W00t! Problem solved!
And you thought the Apple on the back was just for decoration!