What do you mean by " the Amazon website will not allow me to deregister it"?
If you're having issues using the site, you'll need to contact customer support. You should report the lost Kindle to them anyway, so they can prevent it from being re-registered. From any Amazon page, go to Help > Need more help? > Contact Us.
If someone "found' it and reregistered it, you won't be able to deregister it as it's no longer under your control. File a report with the local PD and the place where you lost it is all you can do at that point as it's gone and if you didn't record the serial number you won't even be able to prove it's yours if found.
NOTE: I do NOT speak for or represent Amazon in any fashion. This is a customer to customer forum.
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