This error indicates corruption of your project file. There are 2 methods for fixing database corruption. It is usually best to utilize both before trying to work in the file again. 1. Launch NVivo without opening the file, go to File->Help->Compact and Repair and select your file in the Browse window. 2. Once this completes, open a new, blank project file and import your file into it. Save the file as a new copy of the project.