When did you last log into the mailbox? If you don't log in at least once a year, the account is inactive and purged and the only way to recover it is if you have the password - resetting the password wont work. Logging into outlook.com will reopen the
mailbox, although it will be empty. (The same applies to OneDrive).
If you haven't accessed the Microsoft account in 5 years it will be deleted (closed for good). Note that this changed as of August 30, 2019: Microsoft accounts will be closed after 2 years of inactivity. (See https://go.microsoft.com/fwlink/p/?linkid=2086738
for the new Microsoft account activity policy.)
Once closed, addresses cannot be claimed or reclaimed, or otherwise reopened.
The following is from the services agreement that is in effect as of August 30 2019.
https://www.microsoft.com/en-us/servicesagreement
4. Using the Services & Support.
ii. Account Use. You must use your Microsoft account to keep it active. This means you must sign in at least once in a two-year period to keep your Microsoft account, and associated Services, active, unless a longer period is provided in the Microsoft
account activity policy at https://go.microsoft.com/fwlink/p/?linkid=2086738 or in an offer for a paid portion of the Services. If you don t sign in during this time, we will assume your Microsoft account is inactive and will close it for you. Please see section
4(a)(iv)(2) for the consequences of a closed Microsoft account. You must sign into your Outlook.com inbox and your OneDrive (separately) at least once in a one-year period, otherwise we will close your Outlook.com inbox and your OneDrive for you.