How undisable my Apple ID

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

If you cannot resolve it yourself:

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

If your Apple ID is locked or disabled - Apple Support

Reset your password. If you've done that and it still won't let you download or update apps then you more than likely have a billing issue and need to contact iTunes support.