Hey there! Nice to meet you!
Based on your description, the hacker was able to remove the account's primary email address/security info and replace it with their own email address, which is why you received the error " account doesn't exist" when signing in.
The only way to recover a hacked account is through the official Microsoft troubleshooter guide:
If the account recovery web form doesn't work, and since the security info has been replaced by the hacker, I'm afraid there is no other working method to recover the Microsoft account, and Microsoft Support cannot help either.
Microsoft Support will not be able to assist you as stated in this article, that they are not permitted to send password resets, access, or even change any details on the account:
Thank you for your understanding.
Best regards,
Darhyl