Hey rook2632,
Sorry to hear someone else had gotten into an account you've invested money and time in. If you're still able to sign in to your account through the Xbox link, please try to go through as many of these security steps as possible to help get your account and system back under your control.
After that, or if you're unable to log in, please contact our live support team to recover the account and adjust any inaccurate account information. This process involves discussing private account details, which isn't something we can help with here on the forums. The advisor will ask you a few questions to verify your ownership of the account before helping you make any changes.
Please note that you don't need to be signed in to the impacted account to get help recovering it. Any account, or even a guest login, will do. Let us know how it goes, thanks!