Ganer tag changed

One thing you can do is reset your password and set up a 2 step verification so you can make it hard to crack. As others have mentioned, it could have been automatically changed due to a violation in the code of conduct (check on https://enforcement.xbox.com/en-us)
or that someone has been on your account (in this case go onto https://support.xbox.com/en-NZ/my-account/security/compromised-account-solution).

Hope this helps

Sorry my profile picture has been changed not my gamer tag and the person has claimed they have hacked my account

As long as you have access to the account email, you can change the pw back.

Just to add onto what some of the other suggestions. People can check their account activity

here. From there, you can see when and where you've used your account, and notify Microsoft of any suspicious activity.

One more thing. I would suggest changing your password and enabling two-factor verification if you suspect that someone else has gained access to your account.

https://support.xbox.com/en-CA/my-account/security/turn-off-two-step-verification

It is possible it was a forced change by the XBLPET for a violation of the code of conduct. You can check here for information HTTPS://enforcement.xbox.com