Hi, I know you've had plenty of great advice, but I must just stress the importance of having duplicate files of you work! On your computer have at least 2 (I have 3) copies of your work in different folders, use an external device(s) such as USB memory sticks, and email backups are good too. It's a good idea also to save your work every 10 minutes or so - you never know if your computer is going to crash, or as I've had happen to me, a power cut(that cost me a good 6 hours of hard work). At the end of the day you can't be too carefully when it comes protecting anything of importance....
Some people have luck with save as filtered HTML on Word. A Kindle book is essentially a web page, using very basic HTML language. The Kindle Publishing Guide lists the tags that Kindle recognizes, there are only a few -
The last book I published, I used Word2HTML, a free program that creates a really clean version of HTML - then I went in and added the tags for chapter headings and page breaks at the end of chapters, and lastly, added the links to create a table of contents and the "go to's".
The beauty of this is you do not do any of those weird functions with word that do things like make files disappear and once you know the basics, it's not hard to find problems and fix them.
The only glitch with W2HTML is that sometimes it doesn't close italics tags, so if you like italics, you may spend some time hunting down open tags so your whole book doesn't appear in italics.
If you plan on publishing several books, it's worth an afternoon to learn html basics.