This happens when the logbook gets corrupted, which seems to be quite easy to do. If you tried to edit any other entry than the last one within FSX then it is probably corrupt.
The good news is you can fix it. Go
here and down load the logbook editor. Usually if you delete, or at least edit, the last entry in the bad logbook it will start recording flights again. Make sure you copy your logbook just in case something happens.