Well, if you know what setting you changed that broke it, you could try going directly into the database and changing that setting back, see if that fixes it.
As for the error itself, it's basically saying that there's a bit of code that the script is trying to use, but that function hasn't been defined yet. Usually that happens when you forget to include the file with that function... or it gets included later in the page, and something before that tries to use it. Was that page something you edited/created at all?