How do I undisable my iTunes account?

What you need to do depends upon what message you are seeing. (Including exact error messages when you start a topic helps us help you.)

"If you see a message that says "Your account has been disabled in the App Store and iTunes," there might be a problem with the payment method that you have on file." Click on the link in the document to contact Apple. --> https://support.apple.com/HT208856

OR:

If your Apple ID is locked or disabled - https://support.apple.com/HT204106 - "If you or someone else enters your password, security questions, or other account information incorrectly too many times, your Apple ID automatically locks to protect your security and you can't sign in to any Apple services. You can unlock your Apple ID after you verify your identity."

What to do after [or before] you change your Apple ID [email address] or password - https://support.apple.com/HT204071 - "After you change your Apple ID email address or password, you might need to update your sign-in information with any Apple services you're using." - Or if you're seeing persistent disabled messages or repeated requests for the password to your previous Apple ID.

If you cannot resolve it yourself try one of these contact methods:

"Use this form for assistance with account and billing, code redemption, or purchase questions."

https://www.apple.com/emea/support/itunes/contact.html

Contact Apple for help with Apple ID account security - https://support.apple.com/HT204169

Contact Apple for support and service - "See a list of Apple phone numbers around the world." - https://support.apple.com/HT201232