If you have data to be backed up, I wouldn't be doing any other work on it, other than trying to recover it (see link above)
But once you are happy with not recovering anymore data from it, you can then create a Partition (and format it) on it, which will automatically allow Windows to assign a new drive letter for it
Have a look at Control Panel-->Administrative Tools-->Computer Management-->Disk Management
From there you can Format so forth (only after you have recovered data first!)