Hi. I'm Brian and I'll try to help.
If you can log in without any issues now and you are using a personal account, please verify that you are still the only one accessing your account by visiting https://account.live.com/activity to make sure there are no unrecognized accesses. You should also visit https://account.live.com/names/manage and https://account.live.com/proofs/manage to make sure that you recognize all the information there. Also, enable two-factor authentication or configure a passkey at that second URL. Next you should look at https://outlook.live.com/mail/0/options/mail/rules and https://outlook.live.com/mail/0/options/mail/forwarding to make sure the information there is all recognized as well. Finally, use a password manager to general a long, complex and unique password for the account. You should never use a password for multiple accounts.
If you have another address you can use to send a message to the Microsoft mailbox, do so and see if you get a non-delivery report returned if the message doesn't arrive. If you do get a non-delivery report, the reason for the failure will be in that report.