There is no direct information about the Notes tab in the MyHeritage Family Tree Builder application. The only thing you can do at this time is to attempt a possible recovery, but that doesn't guarantee you'll get your files back. For example, if the notes were stored in other files, but you still have access to those files containing other data, recovery is useless and shouldn't be used. Use Recuva to attempt a possible recovery. This could also be a problem with the software. The notes have been deleted due to the software being corrupted or someone actually deleted them.