If you have forgotten your login or it is no longer valid, etc., you have to contact MS yourself, which can be quite complicated and time-consuming.
Advise everyone to make a note of the MS login, to enter an alternative valid email address and a mobile number, as well as to log in to a Microsoft account (Outlook.com, or Hotmail) from time to time if this is not used as the Windows 10 login, otherwise the email address will be deleted after some time, which then becomes complicated (see above, first sentence).

The licence is stored on the Microsoft Office account. This should not be a problem.
Otherwise, Microsoft Support can be contacted.