When an AppleScript file is opened in Script Editor, there is no text visible to edit.
Solution
If you downloaded the AppleScript 1.4.3 "Z" (English-International) version between 2000-05 and 2000-07, it is likely that you downloaded a damaged file.
The Software Update mechanism automatically downloads the version corresponding to your operating system. If you have an English-International version of Mac OS 9 and used the Software Update control panel to update to AppleScript 1.4.3, you would have received the damaged software.
A corrected version of AppleScript 1.4.3 English-International has been posted. It is available from the Apple Software Updates Web site at http://www.apple.com/swupdates.
Note: Scripts created with the damaged version cannot be recovered; it is as if they have been saved as "run only."