How can I get my computer to start up?

I turned off my iMac at the power outlet as it was shutting down (accidentallly).

I now only have a grey screen and a window with options to: Reinstall OSX, Disc Utility or Online Help.

I've tried all the options...I've tried to start up from my external Hard Drive but it won't accept. I've tried to reinstall OSX Capitan and it won't accept.

This is the first time this has happened to me so I have run out of ideas and there is nowhere local that can repair a Mac.

Can anyone help?