Recovery HD doesn't boot
Just got it fixed by the help of Apple Support following this Apple support page: How to Reset NVRAM on your Mac


  Resetting NVRAM
  
  
  Shut down your Mac.
  Locate the following keys on the keyboard: Command (⌘), Option, P, and R. 
  Turn on your Mac.
  Press and hold the Command-Option-P-R keys immediately after you hear the startup sound.
  Hold these keys until the computer restarts and you hear the startup sound for a second time.
  Release the keys.
  
  
  After resetting NVRAM, you may need to reconfigure settings for
  speaker volume, screen resolution, startup disk selection, and time
  zone information.