Bootcamp Windows 7 install looping

Please help.

iMac 27" late 2011 running Yosemite.

Ran Bootcamp and the installation of Windows 7 started ok. After initial load of setup and installation files, the IMac restarted to boot up into Windows for the first time. At that point, the computer restarted itself, and is now stuck in a loop repeating this.

I'd like to get back to OSX but CMND-R doesn't respond nor do other startup keys combinations.

I Have a Yosemite install USB and a bootable external drive backup created with SuperDuper.

Can anyone help?