My Yahoo email is being hijacked to a “@live.com” account that I dont own

Someone has hijacked my Yahoo email account. Now when anyone hits reply to an email from me the autofill directs the email to a @live.com email address of the same name as my email instead of directing the email to my Yahoo account. I have been able to find the "@live.com" account but cannot access it as I did not set it up & do not know the password or security question. How do I stop this before someone sends something sensitive to the "@live.com" account by mistake? If they have not already.