Message Fizzer, the game creator and site admin, with any evidence you have of that being your account: https://www.warzone.com/Contact
Also, usernames are non-unique (there's over two dozen accounts named "Alex") so you might need to provide more specific information for it to be looked up fast. Not sure. If you have the profile link or a game link in which that account played (or is playing), that might help.
I haven't been through this personally, but Fizzer has a reputation for being responsive and helpful in circumstances like this one.
Edited 7/31/2020 23:53:43