Hello,
If you tried to sign in and received a message that 'We couldn't find an account with that username" or "This account does not exist," it means that your account might have been compromised.
Have you noticed any unusual activity on your Microsoft account, or received any security warning emails from Microsoft regarding your account or security information change?
If your account was compromised and the email address (username) was changed, you will need to submit an account recovery request directly to the Xbox recovery team. They can help you recover the account when its information has been altered and you can no longer sign in to Xbox games.
https://support.xbox.com/en-US/forms/recover-your-microsoft-account