Either the HDD died, or your entire partition table was wiped. Were you doing anything with Disk Utility before this happened?
If it's the HDD, they fail. They're mechanical and it happens. They can last ten years, ten months or ten minutes. So to hear that you had a new one put in six months ago isn't out of the realm of possibility that it went bad. Have you dropped it recently or has it experienced any trauma?
You may need to get a new HDD and hopefully you have all your stuff backed up with Time Machine or another backup software suite.