unable to boot my iMac
Agree with Tetsujin about doing an SMC reset.
I recently had the same issue - I had hard-powered-off the computer in the middle of booting, and my system would then just remain black-screened at boot.  Resetting the SMC brought everything back to normal (able to do Recovery Mode, Choose Boot Disk or boot from internal drive as usual).

However, for me to reset the System Management Controller (SMC), the procedure was different.
From Apple, for computer without removable battery: 


Shut down the computer. 
Plug in the MagSafe power adapter to a power source, connecting it to
the Mac if its not already connected.
On the built-in keyboard, press the (left side) Shift-Control-Option
keys and the power button at the same time.
Release all the keys and the power button at the same time.
Press the power button to turn on the computer.


Note: The LED on the MagSafe power adapter may change states or temporarily turn off when you reset the SMC.

( SMC reset: http://support.apple.com/en-us/HT201295 )

This immediately solved my booting issues (just last week).  Good luck.