This has happened to me three times...
Sadly, it means you have lost all of your save files (Unless you used the cloud)
When it happened to me the first time I had to format the HDD by going onto storage and pressing the button that 'formats' it.
This wipes the entire HDD. However, you can get all of your DLC by going to download history and re-downloading them. (Takes me at least a day) You cannot get your saves back though
(Again, unless you used the cloud)
When it happened a second time i re downloaded everything AGAIN, but this time I switched to the cloud. After a while, it happened a third time and I lost everything except for my saves.
So I decided to buy a new HDD and everything is better... For now.
So what I'm trying to say is...
Solution=Buy a new HDD