Hello, thank you for reaching out to Microsoft Q&A.
It appears that your account is being hacked/compromised.
That is, the hacker has gained access to your account, removed the email address (Primary alias) from your Microsoft account, and replaced it with their own address.
If this happens, you will get a 'This username doesn't exist' error message during sign-in, and account recovery becomes significantly more difficult, as the attacker may have altered all account-associated information.
However, I recommend submitting the Microsoft account recovery form using the link below. Fill in all the details you remember and submit the form.
https://support.xbox.com/en-US/forms/recover-your-microsoft-account
Also see,