Net galley posted something about this on Facebook. It has to do with Amazon and something in their updated programming. They are working on getting it fixed so that you can download to you Kindle again. This is what they sent out in email earlier today
Due to changes in the way Amazon's personal document delivery service works, we've had to temporarily disable the Kindle button.
All of our other reading options will still be available, in accordance with what the publisher enables for each particular title.
Stay tuned; we'll be working on a way to reintroduce this reading option just as soon as we can!
Hope this helps!