All scripts in Celtx have a history. It's on the top right of the script. Each time your document saves, the history of saves is restored. So that really horrible line you wrote and since edited out...well, it still exists back there somewhere. But so does that awesome line you accidentally deleted.
Just click on the history and try to locate the approximate date and time you're looking for. The history runs most recent at the top to earliest at the bottom.
If you're in a free trial or have a paid subscription, you can restore the entire script from the history. Or you can just copy the part you need!
The history also serves as an excellent restore point if for any reason the current version of your script is not loading. If you're using a Basic account, just copy/paste!