This is an oddball issue. As in an event that is extremely unlikely, but not impossible.
Like Narelle said, if you login to the wrong server, you’ll see character creation. Also if you login to the right character but the wrong account, similar result.
If its not the wrong server and not the wrong account. There is one thing that could trigger this. If the server crashed and loaded from a backup prior to you creating the character then your character would be lost in that case.
This isn’t common and I’ve never actually heard of it happen. But it is potentially the case. How long did you play that character for? I assume a decent amount of time since you’re level 20 on your first character.
But if it was the span of say… an hour or so. Rather than several hours, this might have chance of being the culprit. Understand that this was extremely bad luck that causes this situation and like I’ve said, in the years I’ve played this would be the first instance of it happening that I’ve heard of. And its speculation at that.
I do think it is highly unlikely though, and its more likely you aren’t connecting to the server you started your character on. As others have said, the ingame browser isn’t very reliable with its filters. You pretty much need to write down the server number when you first connect to it to be sure. The names never change (afaik).