imac is not booting up. Tried all options

All of a sudden, my imac got frozen. Once manually turned off/on in order to reboot the computer, its not going anywhere after apple logo. Tried with all including re-install OS X but still no luck. No issue found in diagnosis, HD utility run. The last option that I have not tried is to erase all existing files, settings etc and install OS X again. Can you please assist what option I have?