Ho do I recover lost books in Kindle app?

Kindle asked me to do an update and I did it.
Then I rebooted the computer and I lost more than 30 books.
Then I asked explained a kindle expert the problem and he told me to uninstall kindle and then reinstall it.
I did that.
Then I rebooted the computer and I lost another 5 books.
How do I:

1. fix the problem?
2. get all my books back?

Thanks