aww man! That happened to me too. As soon as I hit save, I didn't realize that db was uploading it. I tried saving again, Flash crashed, and my file got currupt.
Things I did for all future projects
1. Go to dropbox/preferences/ bandwidth and change 'Upload rate' to "Don't limit". This helped it save much faster
2. Learn a SVN software **like TortoiseSVN** never again have the trouble of corrupt files because of version history taken care off by the software
Psyh
Well I'm learning through other people's errors...
For my first flash I'll remember to have 3000 backup files.
Kiigen
I MAY be wrong, but I think It's cause I had it saved to DropBox. If you keep backups, don't work on the file that's immediately being updated as soon as it changes. It's good to backup... it's why I did it. It also has helped... just don't do that unless you know your PC won't crash before update is done.
Again I MAY be wrong. I have no idea why the FLA crashed.