Maybe this would have helped the other 2 times but I had a similar experience this morning. Last night I had to Force Quit BookWright and instead of reopening it, I shut down the computer and when I started it up this morning it wouldn't boot. It wouldn't even let me get to the point to reinstall the OS. It was showing that I needed new batteries for my keyboard/mouse. I recently replaced the mouse batteries but maybe the reason I couldn't get to the reboot was because the keyboard wasn't working. While I was replacing the batteries and seeing what I could do, I used my laptop to look up why I couldn't get the iMac to boot. I came across this http://support.apple.com/kb/TS2570 and it did the trick.
At first I tried the Soft Boot by holding down the shift key and then it went back to the gray screen with the apple and spinning globe so I shut it off to try the boot where I could see what was going on by holding Shift-Command-V during start up. I liked this much better because it wasn't just looking at a screen, you saw words on what looked like an old DOS type screen.
7/3/14
Thanks to BookWright and maybe my being on it for too long at a time, I've had to reinstall OS X twice in less than 24 hours. Thought I better put these instructions here in case I forget.
