I found my Kindle for PC not working as well. didn't know why until I found this discussion. I did not receive any notification from Amazon and very unhappy about this.
All my e-books seem to be available though, including some I obtained from non-Amazon sources.
It would be helpful if Amazon could be bothered to let people know when they make changes which affect everyone with Kindle for pc.
I've also got Kindle on my phone, will the same thing happen if they update that as well? Just not acceptable.