Hacked Account and Security Information Changed

This may help you How to recover a hacked Microsoft account

this will not work if they have already set up 2 step verification.

You can use the account recovery form to regain access to your account.

Information on what you need before using it can be found here.

I would highly suggest contacting the online support team and talking to an agent

I'd suggest adding additional security information to your account if you believe you've been hacked and someone is attempting to use an alternative email address.

But first contact xbox support.

Here's some info on how to change your MS account email

And here's some info about account security link