Missing character
Three possibilities come to mind:
1. You were hacked quite a while ago, and the hacker deleted your character after messing with stuff.
2. You shared your account info with someone else who deleted the character.
3. You deleted the character and, in the long period of time you were away from the game, forgot.
Those options seem more likely than some GM moving your character to another account and changing the name (something that, as far as I know, has never happened in the history of the game).
Regardless, if you’re unhappy with the response from support, you can update your ticket asking that your issue be reviewed again.
Agree with RoseofGilead here you sure your login info wasent saved and a family member played a prank on you?
Besides, you should still have all your achievements at least, since those are account bound.
If you remember the name or can find out from something soulbound in the bank that would help.