Hi there! My name is*****'m sorry that you are having this problem. I will do my best to help. The best thing to do if you do not know the pin is to reset the Kindle Fire back to Factory defaults, just as it was when it came out of the box.
Remove the charger from the Kindle and hold the power button or switch for a full 40 seconds. This should reset the Kindle to factory defaults. You will know that it successfully reset to factory defaults, if it asks you to go through the setup process (providing Amazon account info and connecting to wifi) when it restarts. Did it do that? If not, please try it again, being sure to hold the power button for a FULL 40 seconds.
Please give this a try and let me know how it works for you. If this solves the problem, would you mind taking a moment to rate my service? You can give your rating by selecting the stars located on the top right of the question page. I'd really appreciate it! Without a positive rating I am not credited for my answer. If this doesn't solve the problem be sure to let me know so we can continue troubleshooting.