If you checked in-game someone most likely broke into your account. Check here for IPs that aren't yours.
If you just checked on the website, most likely flagged for a rename so you can't see them.
But as I explained earlier, when I log in in game there is no characters nor any information that would suggest that I have to rename characters.
They are just missing.
I've tried logging into every server available and there is nothing.
This means you were playing ONLY on Frostmourne relam... (then why your title is "Missing characters from all realms " )
you were playing on "Frostmourne" realm during "Season 1"
Which started on 15-April-2020 and ended 14-April-2021
This was the Thread about Season 1: http://forum.warmane.com/showthread.php?t=415559
You can still play with your old character from "Season 1" by transfering him to "Icecrown" realm https://www.warmane.com/account -> service
Select "Frostmourne - S1" as realm