All my archived items have just disappeared from my Kindle E reader (3rd generation). I had about 300 items archived, but now none! Even archiving a book I've just read, has no effect. It just isn't stored as before
Go into your account then "content and devices" all your books should be there and you can resend them to your Kindle, I assume you have not changed your email address and accidentally set up a different account.
Also look at your settings as if you have archived the books and you have the Kindle set to "downloaded" you will not see them and will need to switch to "all"
BUT First hold down the on/off button on your Kindle for 30 seconds and let it reset, your books may come back.