For future reference the standard proceedure when encountering a corrupt mission is as follows.
1. In your missions folder you'll find 8 files called Backup.00x. Make a copy of these immediately. Do
nothing else until you have done that.
2. Rename your mission.bak file to somethingelse.fs2
3. Load it in FRED
4. If that doesn't work, work your way through the backup.00x files you made copies of earlier. Be warned that FRED will make new ones each time you try to load a mission, this is why it was so important to make copies before did anything else.
5. If there are significant changes between the mission you managed to recover and the version that became corrupted you may wish to back them both up and then open them both in
WinMerge or whatever diff program you like. Sometimes the corruption is really obvious. And even if it isn't you can often copy across large portions of your work at a time until you figure out what caused the problem.