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.
I've not dropped it or done anything with the disk utility. It was working an hour ago. Do you think my local mac store will be able to do anything about it? Anything of value is backed up with iCloud but I could really do without spending any serious cash at the moment. Also got a fair few movies on this thing which haven't been backed up and not purchased through iTunes