Apparently, there's an error with an "If" statement, and now I can't open the editor. Help?

Eh... this shouldn't happen. Can you give the error message?

And what exactly is the problem? You can't launch the Quest Desktop application at all, you can't just edit a given game, or you can't test it?

As an emergency fix for the last two cases, open your game with an editor (Notepad++ or the like) and try commenting out every if you can find.

This has happened to me a few times.
In my case, it was caused by altering code in 'code view' and then either saving the game or going back into editor 'GUI' mode.
Try looking at the game in a text editor program as has been suggested and deleting/altering the last piece of code you were working on.
And in the case of an 'If' statement usually the omission of one or more '}'.