Hi,
Based on your explanation of where you are getting the "Account does not exist" error and your observation that the email address has likely been changed, this indicates that your account was hacked, and the hacker changed your email address.
If the hacker changes your email address and other information, recovering your account can be challenging, and it may be permanently lost. I understand that this is a difficult situation. You can check the Microsoft article below to see if it can help.