Hi,
Based on the information provided, it is possible that your account has been hacked. The hacker may have signed you out everywhere and then immediately changed your email address, which is why it shows that it no longer exists when you try to log in.
Please note that if your email account ends in @hotmail, @outlook, @live, or @msn and was removed, replaced, or deleted due to inactivity, it cannot be used to create a new account. See the “Remove an email address” section here: https://support.microsoft.com/office/add-or-remove-an-email-alias-in-outlook-com-459b1989-356d-40fa-a689-8f285b13f1f2
If your account has been hacked, please use the sign-in helper tool in the link below to guide you to the appropriate solutions. The link provides comprehensive steps to help you recover a hacked account and also gives you the option to chat with Microsoft Support as a last resort:
https://go.microsoft.com/fwlink/?linkid=2196140
Also, please fill out the account recovery form here: https://account.live.com/acsr
Microsoft takes account security seriously, so the recovery process is fully automated to prevent unauthorized access. Please complete the form using a device and location you have used before, and provide an accessible email address.
Microsoft usually reviews requests within 24 hours. If approved, you will receive instructions to recover your account.
You can submit the form up to two times within a 24-hour period and include any new details you remember each time.
If none of these steps work, I’m sorry to say that if the username has already been changed to a different one, it may indicate that the account has been taken over by the hacker. In such cases, recovery may not be possible and the account could be permanently lost.