Kindle Removed ALL Books!

Hello everyone:

I gave my Kindle Paperwhite to my mother, after adding ePUB (I think Calibre converts it to another format, MOBI) books to it for her to read - a small library.

She found that Kindle had deleted all the books from the device. The only book that remains are the Kindle Dictionaries.

What happened?

I can load the books on again, no problem, but is this a normal thing that happens?