Revert to last Gamertag changed by someone

May I ask how someone had access to your account to change your Gamertag?

Also, you can change your gamertag, usually with a $10 fee if you have already did so once already.

More info, and how to do so by various devices can be found here: https://support.xbox.com/en-CA/help/account-profile/profile/change-xbox-live-gamertag

Also note, there is a new Gamertag system where you can only use 12 characters which will be followed by a randomly generated 4 digit number at the end, similar to Discord and Battle Net.

However, since someone else changed your tag without your knowledge, I would recommend contacting Support directly. There available times and methods of reaching them is also available in that link provided.

Hope this helps you out!

My data has been deleted in i need to talk to Microsoft asap

If someone takes your gamertag before you change it back or you try to change it before the server notices it's freed up, then you would get a suffix.

the suffix will be removed, but why revert back?

Unfortunately the only way to change it again is to pay for it.