iMac won't boot

I have a 2011 27" iMac that won't boot up, when I try to hold down the option button on startup it will not recognize the hard drive. It will see a usb drive and a DVD but will not boot from them either. I have verified that they are good drives on another mac. After that I tried to use it in target mode and the host mac wouldn't see it either. I have replaced the hard drive it's self and also replaced the ram. I am at a loss as what to do next. Thank you any help