Oh my, so sorry to hear that. I hope you find your books.
Happened to me somehow (not sure if I pressed wrong buttons or what
) but the books disappeared. Thankfully it was my first few freebies so I easily downloaded them again. Now I have started a 'backup' library - before I transfer to the reader, I copy the books into another folder on puter.
And unfortunately if you work with WIndows, anything is possible, including the sudden and mysterious disappearance of files.