The NLS control information for a file in your program does not match the same NLS control information in the header of your index file. Alternatively, your index file has become corrupted.
Rebuild your index file, or rerun your program using the backup copy of that file. If you have added a great deal of information since you last took a backup, you might want to rebuild the file using a utility that is able to read the data, if it is not corrupt, and build a new index for it.