You have to see if it's recognized, even non formated in windows disk management (execute, diskmgmt.msc, enter). If yes, try loading easeus or active partition recovery to make an image (if possible) or recover files right away. Alternatively, try Partition Find and Mount to see if the former partition is seen, then backup files asap.
Sometimes, a few bad blocks in the partition table cause just that. Try also running HDTune to see if there are some physical errors. If yes, try imaging the drive before it degrades further. Then use something like spinrite, HDD Regen (demo version) or DRevitalize to try to repair the bad blocks.