It could be that the partition that was then unknown (No FS, No Drive letter) just got it's Partition Table corrupted because of some reason. What kind of reason is hard to disclose. But in this case, some special software should be able to restore the data that was (is!) on it. EaseUS Partition Recovery for example.
Hope that helps a bit!
A Link to EaseUS: http://www.easeus.com/resource/partition-recovery.htm