Originally Posted by dwig
I can't say why this happened (database corruption, accidental reset, ... ??), but your K3 seems to no longer be "registered" to the Kindle account from which the books were purchased and the books' DRM is preventing them from being read. Simply re-registering the K3 should heal the problem.
I've had it happen with both DRMed books and non-DRMed books. This one was a side-loaded KF8 file converted from epub with kindlegen.