Hi, I'm John! I will help you with this.
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.
Click "Add comment" to let me know.
Regards,
John J.D.