"If you cannot remember the password for your Kindle, you can reset your password to regain access. Resetting your password will delete all content on your Kindle and you'll need to register your Kindle again before downloading items from your Kindle Library on Amazon.com.
To reset your Kindle if you don't have the password:
Slide and release the power switch to turn on the device or exit sleep mode.
Enter "resetmykindle" in the password field and press the enter key on the key pad.
Wait several minutes while your Kindle restarts."
How to reset Kindle password?
You can press down on the 5-way controller to see your password hint. If you still have trouble with your Kindle password, you should contact customer support phone line and they will help you reset it.
If this happens to be a stolen kindle, and it is reported to amazon as such, the device will be blocked from many functions. Anytime someone forgets a password, there is a chance it's stolen. Good luck.