Have you done something improper to this drive?
1). Are you intentionally hiding these files?
If so, just try to unhidden them: Start > Run > Type in "control folders" > View > “Show hidden files and folders”.
2). Is your drive virus-infected?
Scan your drive with the updated antivirus tool in your computer and find your needed files back. Take care not to delete your wanted files.
3). Try to use data recovery software?
If these two methods above could not resolve your problems, a data recovery programs could be a good chance.
If you have no idea about data recovery stuff, you could try to download some drive data recovery freeware to take chances.
Just read more information of drive data recovery freeware here: http://blog4mark.blogspot.com/2012/10/only-part-of-drive-data-can-be-accessed.html
In your case, you could be much easier to get back your wanted files, if you have a data backup. So, just never forget to make backups of everything important from now on.