One sure way to retrieve your file or folder attributes back to unhidden is to use command prompt in windows. All you have to do is described below in steps.
Plug your pendrive to USB port of your computer. Make sure it is detected.
Start command prompt by Click Start>>Run and type cmd then hit enter.
Find the drive letter for the conected USB drive. For example, G:
In command prompt, type G:
attrib -s -h /s /d *.*
Make sure that you put space between each elements in the code.
hit enter, wait a moment and this should unhide all your files on your pendrive. Checkout the screenshot for more to clarify.