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Someone definitely put that on your Kindle if it isn't you. First enter some incorrect passwords for 5 times in a row and it will ask you to reset it. Tap the orange link that says Reset Your Parental Controls Password. If you don't see this link on your screen, you may need to scroll down in the message window. Enter your Amazon account password ***** create a new password.
If that didn't work, then try registering the device from online. Follow the steps below;
If that didn't work, then you have to reset your Kindle to factory settings unfortunately. This will remove everything from the Kindle, but you can redownload your books and purchased contents back to it after re-registering with the same Amazon account you used on it. Follow the steps below to factory reset it.
(If asked for password, try resetmykindle
Please let me know if that didn't work, so we can continue troubleshooting.