Try a reset, Fiona - that's the restart where you hold the "off" button for a count of 20-30 seconds, up to the point where the screen goes blank, then flashes and starts to reload. Leave it for a few minutes while it reboots (just like a computer) and you should find your archive books have all come back. If not, though, you should still be able to re-download them via Manage Your Kindle on your computer.