Stopped playing for like, 2 months, lost my name

Hi, I've been playing this game for years, and I recently stopped for a bit and decided to come back today only to find that the username on my Main Character, that I've used for years now, is no longer mine. I want it back. I've put so much money and time into this game, and I want to be able to use the Nickname I've been using for a decade in real life, I don't know what happened, or why I had to reactivate everyone of my characters. Can someone help?