Hey there! Nice to meet you!
Based on your description, the hacker was able to remove your account's primary email/security info email and replace it with the hacker's email, which is why you received the error "account does not exist" upon signing in.
The only way to recover a hacked account is through the official Microsoft troubleshooter guide:
If the account recovery webform doesn't work, and since the email address has been replaced by the hacker, I'm afraid there is no other working method to recover the Microsoft account, and Microsoft Support cannot help either.
Thank you for your kindness and understanding.
Best regards,
Darhyl