Read this support article --> How to generate a recovery key - Apple Support
If the thief was able to change your account information, the thief likely also set a recovery key which means features such as account recovery are unfortunately not available to you. Contact Apple and see if they can do anything about this but from what the article says I would not be optimistic.
Contact Apple for help with Apple ID account security - Apple Support
"This article provides country-specific Apple Support contact information for customers seeking help with their Apple ID password or other security-related issues."