Yesterday, one of my colleague's USB infected by virus. It made every folder in USB shown with exe extension. After scanning with anti-virus, all those folders were disappeared. But somehow, virus made it hidden. I sufted the web several times to unhide them, but unsuccessful. Later on, I found this command "attrib" . To unhide everything in your folder:
Go to Start->Run-> type "cmd" -> type "attrib I:\*.* -s -h /s /d"-> enter
In some computer USB drive might not be "I", So the command should change accordingly.