"I somehow missed that this was happening a few months back, so maybe other people did, too.
My Kindle for PC updated itself silently to version 1.19, which is compatible with Amazon's latest formatting. This is great for being able to use the new formatting! This is not so great if you use Calibre to manage your library. Calibre can't read or even recognize the new format, so can't import your books.
If you use Calibre, the easiest thing to do is to uninstall Kindle 1.19 and reinstall the older version, 1.17, and make sure that the "automatically update" box is unchecked.