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This usually happens if the firmware is glitched. It is very unlikely to be the card issue. First we can can soft-reset the Kindle and see if it works or not. Press and hold down (or slide) the Power button for 20 seconds and this will reset the Kindle. Then check if you are able to unlock it.
Please let me know if that didn't work, so we can continue troubleshooting.
Do these kindles need new batteries sometimes? Not unless the device is not turning on at all.
Even when plugged in to current there is no change. Can you suggest something? Yes, reset the Kindle using my above instructions. If that didn't work, then connect it to your PC and then reset it while connected.
Do I need to purchase another Kindle? If the Kindle is not working, then you can either send it to Amazon for a replacement for a reduced charge or you can buy new one.
I have had 3 now-the first was replaced by insurance and was the earliest model. That one has a library of books I want to keep, so I got the Fire model. is therea limit to what they can hold? That depends up on the number of books and size of the books. But you don't have to worry. You can delete old books to redownload if they run out of space. Deleted books from the device can be redownloaded to any device at any time you want from your Amazon account. You will not lose any books you purchased even if you replaced the Kindle.