When a Gradebook has been significantly altered, going back to a previously saved version may be the best option.
Use the Restore Saved Grades option from the Gradebook Tools menu. Even if an earlier version is restored, you can still restore back to a more recent version.
Note: DO NOT use this feature to recover a Gradebook you've accidentally deleted. The restore grades tool is to be used only with Gradebooks that have not been deleted. To restore a Gradebook, simply create a new one with the appropriate date range. Follow these directions to add a Gradebook.