sometimes especially external disks get “lost” using windows. The Windows Disk Tools mostly are not able to recover such a disk and suggest to format it as it isn't formatet. but in many cases the only fault is a corrupt superblock or an inconsistent FAT.

So an approach for recovering – besides professional tools like kroll ontrack – is to use the linux tools for NTFS:

• Install ntfs-3g with sudo apt-get install ntfs-3g.
• Then run the ntfsfix command on your NTFS partition.
• For example:

ntfsfix /dev/hda6