Have you tried downloading any books recently? Last month Amazon withdrew support for some of the earlier models. Here's what they said:
"Thank you for continuing to use one of our earliest Kindle E-readers. While you can continue reading on your E-reader, as of August 17, 2022, store functionality will no longer be available. This change only affects certain E-readers introduced 10+ years ago (listed below). As of the 17 August you will no longer be able to browse, buy or borrow books directly from these Kindle E-readers. As always, you'll be able to browse, buy and borrow books on other supported devices or through amazon.co.uk/ebooks.
Which Kindle devices are affected?
Kindle (2nd generation) International, Kindle DX International, Kindle Keyboard, Kindle (4th generation) and Kindle (5th generation)."
Mrs NJ has one which sounds like yours and hers is one of those affected by the above.