1. Open Computer and note the drive letter for the pen drive (example F
2. Go to Start > Run > type "cmd" (without quotes)
3. Type the following in the command prompt:
attrib -h -r -s F:*.* /s /d (where F: is your drive letter)
4. Exit and check if the files / folders are there
It must be due to removal of volume(i.e. removable device) letter, just visit computer management and check for ur removable media drive letter, if already assigned a value, change the letter and try again..