Sending, receiving, and organizing email in Outlook.com
Hi there! Just to confirm, have you also checked if there are any information added on your account that you are not familiar with? Such as an unknown email address, phone number, etc. First thing you should check is your accounts security information via https://account.microsoft.com/security page.
From there, check if your recovery information is still up to date or if someone put theirs on that page. If you see an information you are not familiar with then it is likely that the hacker changed your account information and that they may still have access to your account.
If that's the case, best is to go through the hacked account guide of Microsoft on this page https://support.microsoft.com/en-us/accounts-billing/manage/how-to-recover-a-hacked-or-compromised-microsoft-account by following it you should reach a form that will allow you to submit a report directly to Microsoft. I hope the additional information helps!