It sounds like that some virus may attack your hard drive. Actually, the virus sometimes will hide your files and mark them as file system. So, you cannot find them normally.
Therefore, to save your files, you could try to clean the virus.
But, if the cleaning stuff doesn't work, you are supposed to try a hard drive recovery freeware
, which has helped recover my files from an inaccessible drive. It is quite useful and really free.Note
1. Do not store any new file on this drive to avoid the original data overwritten by new data, which can make your data gone permanently.
2. Do not save the recovered files on the same drive in case of recovery failure.
3. Do not forget to format this drive to see whether it could work normally as before.
4. Do not forget to back up your important files as possible as you can in the future.