This bug often happens when a script is copied and pasted from another source, bringing in some sort of hidden characters. Check out this tutorial on how to import to prevent this sort of thing.
Fortunately, if this does happen, it's an extremely easy fix for you. All you need to do is click the History button in the top-right corner of your script, select the previous time/date stamp in the list and click Restore!
If you have a free account, just copy your older history version, return to the working copy, and paste over your problem file.