If your flash disk got a virus in it. And suddenly your files and your folders are gone, but the capacity of your flash disk is the same when the folders are there. so it mean that your folder or files are being hide by some virus.
how to unhide it? here it is:
1. open windows command prompt.
2. then type your flash disk directory in win command prompt, for e.g.: E: then enter.
3. after that, type attrib -s -h /s /d then press enter.
4. after this process all of your folders and files will come back.