How can I secure or recover the information on a locked iPad device?

I followed instructions to set up my new iPad. I was instructed to get a code from old iPad, or mark approval, I can’t remember which. After I did that the old device is locked and no longer usable. I need to continue using

the old iPad because it has some important information on it. How can I unlock the original iPad without losing its contents?

[Re-Titled by Moderator]