Security questions
I chose letters instead of words for the security questions and now they don't work, letters like ZZZ ,etc. I did that over a year ago
See if you can reset them : If you forgot the answers to your Apple ID security questions
If you can’t reset them contact Support in the country where you are (and which is therefore the country on your account) and see if they can reset them for you : Contact Apple for help with Apple ID account security
If your country isn't on that page try this form to contact Support : https://www.apple.com/emea/support/itunes/contact.html