Google “raw file system recovery”, you may find some similar phenomenon that also happen to you, say, CHKDSK: "RAW filesystem" message, "Disk is not formatted. Do you want to format now?" "Sector not found" messages. You may be surprised to ask why so many people have suffered such computer trouble and want to know what causes it. Most importantly, recovering data from the raw filesystem is just what you desire to know.
While in order to avoid the same trouble again, we need to know some basics first about it. The filesystem type is recorded at least in two separate places: in the partition table and the volume boot sector. Actually, both partition table and volume boot sector contain the consistent information of filesystem type. This is how the things are supposed to be when the volume is online and operational.
As far as raw filesystem is concerned, a sudden reboot (power failure or stop error) or virus attack will lead to such problem and you cannot access the files from the raw filesystem instead, so the most efficient and safe way is to turn to professional data recovery software, which will offer your professional help and do raw filesystem recovery with simple clicks.