My Outlook email account has been hacked

Login to your Outlook web portal and change the password.

Since this is an Outlook problem, you should probably contact Microsoft about it. Outlook is their baby.

Please see > How to get support for Outlook.com - support.microsoft

Been there, done that. But it was the web account that actually got hacked. There's little you can do other than send an email to all your contacts explaining, but likely they already understand. That and change the password of your actual Outlook account to a much stronger one.