Hi, I'm John!
To help narrow down the issue:
Do you remember receiving any security alert before this happened?
Also, please try searching for the account using these steps from Microsoft: https://support.microsoft.com/account-billing/forgotten-your-microsoft-account-username-b2049472-3b8f-27d3-61c6-67a668453f4c Click on "Search" and follow the instructions provided.
If you receive any error messages, such as “account doesn’t exist” or anything else, please let me know.
One important thing:
If an old @hotmail.com, @outlook.com, or @live.com address is closed due to hacking or getting deleted as alias, it cannot be recreated again for security reasons.
Click "Add comment" to let me know.
Regards,
John J.D.