This bug is caused by empty layers, which due to a bug in Flash causes the file not to be openable by CS5.
Fear not! It can be fixed!
rename the .fla to .zip. Unpack the zip. Open it in an editor that can search multiple files and search for
Then, you can open the .xfl file in the root, and save as a new .fla
Credit for this solution goes to http://blog.jasperstocker.com/ but on that blog the node didn’t get rendered in chrome so I thought it worth restating.
I’m really impressed that the .fla format is not actually a binary – I had no idea! An excellent move by Adobe. Still, this bug is a large oversight that should have been picked up before CS5 went to market.