Hello, I’m Nathan, and I’ll be assisting you today. My livelihood depends on your positive feedback, so your satisfaction is my highest priority.
Sorry to hear you’re having trouble, Allan.
What you'll need to do is go into your kindle account online, from your computer.
On this page, click on the DEVICE tab at the top left,
choose your kindle from the list, then click on Deregister.
This will cause the kindle to be removed from your account. If someone tries to use it, or it gets onto a wifi network, it will remove all your content from the kindle and they wouldn't be able to access any of your account info.
This initial step would also allow the kindle to be reconnected to your account if it is found/returned.
If you truly believe it was stolen, and you'll never get it back, you should also call kindle support at 866\321/8851 to report it as stolen. They can put a lock on it, so that no one can activate it under any other account in the future either, rendering it useless to whomever took it.