My husband outlook account has been changed by some people.

This morning, my husband receives a "failed to log in" message from his mobile phone on his outlook.com.  He tries to log in but same messages appear.  This relates to his outlook.com email.  He went to his alternate email and saw that there were some prompt on his alternate email that he has requested for a changed in his email address which he did not.  He tries to follow the instructions from the alternate email there but it keep asking my husband to log onto Microsoft outlook and again there is a prompt stating that his email is not valid.   He does not know what to do now as he cannot access his email at all.

Please assist