A few days ago, after unplugging my Amazon Kindle from my computer, I switched it on and to my horror - the device was empty! All my clippings, all my books and PDFs transferred via USB onto the reader - gone! Just the default manual and the dictionaries where there. But then I thought: hey, I've got "Sync my Clipping and stuff into Amazon's Cloud" switched on, so all my stuff should be safe in the cloud, right? I went to kindle.amazon.com and found an empty account! No backups!!
After this second shock, I thought about it again: what if I plugged my Kindle into USB and looked around the device while in dumb USB storage-unit mode? So I did and found: all my files, clipping, etc. still there! After a sigh of relief, I unplugged the Kindle and had another look at it from the Kindle display: still empty. Puzzling...
Searching around the web brought no insight, apart from the new discovery, that you can actually reset the Kindle. So switching it on and off seems to be different from actually having it boot up, it's more like a suspend mode or so. So I found how to reset the Kindle: by holding the on/off button really long. And a-ha, here was this apple-guy-under-the-tree logo again. And after a while, the Kindle was up again and all my books reappeared!
So, as it turned out, the Kindle had crashed. Probably because I had the device connected via USB for charging while I booted my PC several times. Or due to really low battery. Or both. Anyway. Kindle doesn't seem to like this and resorted into stubborn default mode. So everyone reading this here, with a similar "Kindle books have vanished" symptom:
Reset/reboot your Kindle by holding the on/off button really long!