My iMac is stuck after downloading Yosemite and opening the install. It is stuck on the dark grey screen with white box in the center labeled Install OS X with the X in a circle below the text. Status bar shows all the way blue, but no prompts. I've tried shutting down, even unplugging for several days before restarting, and still stuck on the same screen. Help! This computer is essential for school. Any words of wisdom would be appreciated!