Wow…(to coin a phrase)…
I’ve been playing since vanilla, have a full staff of characters on several servers- I go to log in as normal (since they’re billing me monthly) and ALL my characters are gone.
Now, ordinarily, you’d contact support and maybe get help, but guess what? The help pages won’t load, that’s what.
My first try actually produced a response, which was immediately marked “resolved,” but without a resolution.
If you have multiple, have you ensured you’re logging into the correct license above the Play button on the Launcher? Sometimes it likes to choose one at random.
To add to what Ekon said, making sure you’re logged into the right WoW account, the correct version of the game (Classic, Live, etc.) and that you’re also using the correct e-mail address.
If it’s been an extremely long time since you logged in, you might need to make a character on the realm where you remember having characters in order to get the list of characters to populate too.