Do you have a Kindle from before 2013? That includes the first-generation Kindle Paperwhite. If you missed the March 22 yesterday's deadline for a software update, it may have stopped working properly.
Customers using an outdated software version on Kindle e-readers require an important software update by March 22, 2016 in order to continue to download Kindle books from the Cloud, access the Kindle Store, and use other Kindle services on their device.
What does this mean? Any books you've actually downloaded to your Kindle will still be there in your "on device" library. But the rest of your books won't be, and you won't have access to them. Any magazine or newspaper subscriptions won't arrive, and if you use your Kindle for other cloud-based tasks such as reading email, that won't work either.
If you're using your Kindle on a regular basis, it's possible the update might have happened automatically. Then again, some users with older Kindles are reporting that their always-connected Kindles aren't telling them they received the update. If you haven't been notified by your device that it's up to date, you can look here to see the software version your specific Kindle needs. If you don't know how to tell which version you already have, there are links to instructions for each device at the bottom of the notice that will help you find out.
An unhelpful message
What if you missed this deadline? Then things get misleading and complicated. According to Amazon's notice, you should see this message on your device:
Your Kindle is unable to connect at this time. Please make sure you are within wireless range and try again. If the problem persists, please restart your Kindle from the Menu in Settings and try again.
That, unfortunately, is extremely misleading because following the steps in the message won't help you in the slightest. According to the Amazon warning, if you see this message, ignore what it's telling you. You'll have to do a manual update to fix your Kindle. That means downloading the new software to your computer, connecting the Kindle to it with a USB cable, and then transferring the new software onto your device. (More detailed instructions here.)
Why is your Kindle giving you the wrong instructions? That's probably a default that Amazon couldn't change without actually doing the update that would fix the problem and render the message moot. Not much mystery there.
But--why is Amazon causing all this havoc with its urgent critical update? That's a much more interesting question, and one the company is not answering. Generally speaking, when software providers urgently issue a critical update, it means they've learned about a major security vulnerability. They might not want to tell the world about it, though, not only because it makes them look bad, but also because it invites hackers to try to find and exploit that vulnerability.
No idea whether that applies here--Amazon may have other reasons it wants to get everyone on newer software pronto. Feel free to speculate--while you're updating your Kindle.