Ctrl+Z or Undo in WE works like in any other program out there. Once you save the changes and exit the program, you can never undo it.
Sorry but you'll have to redo the entire trigger or download the resource again (if it is from the internet).
The thing that wonders me is that how did you accidentally convert the trigger to custom text. When you click the convert to custom text thing, there SHOULD be a warning about not being able to convert it to custom text once again. Did you not read that?