Kindle app not connected to internet

the Kindle app on my iPad has suddenly decided to say it's not connected to the internet. I can read books but any book that I buy or want to download just gives me the no internet message. Strangely, my iPhone is doing the same thing Any ideas? I've restarted, deleted the app and restarted, downloaded the app again from the App Store. It worked a while yesterday but it' s just gone again.