I searched the net when I had the problem and I found this helpful, just sharing.
1. Rename your file to a zip file (something.fla = something.zip)
2. Open the archive then Tools > Repair Archive > Treat the Corrupt Archive as ZIP
3. Rename the rebuilt ZIP file into FLA file (rebuilt_something.zip = rebuilt_something.fla)
Good as new, there might be some inconsistencies though. Enjoy your animation!
Additional Notes by Jeff:
A couple of notes to make this tutorial better:
- This doesn't work on FLA files prior to (I believe) CS5. CS5 is when adobe changes the FLA file format to be based on zip.
- The exact program you use to open zip files is not specified, nor is the OS you are using. On Windows alone there are at least 2 major programs that deal with archives (7Zip & WinRar), along with Window's own built in zip support. Please be more specific.