Macintosh HD not recognized

I wanted to downgrade my iMac form El caption to mountain lion (because I need os x 10.8 for my interface)

At during of restoring hang up and after that when I turn on it was too slow starting up and when loading for startup finished after that turn off!!!!

I restart IMac and hold “option “ key and choose recovery hd ,when I open disk Utility just loading but don’t recognized the Macintosh hd and just loading

And when I want to restore, after detecting time Machin external hard drive, can’t recognized Macintosh hard drive

And is happen when I want to reinstall for new os x also don’t recognized Macintosh Hd

I don’t know what should I do?

Thanks you for your kindness