That means those raw Dicom image files along with the folder had been deleted without involving PacsOne Server, because PacsOne will never delete those raw Dicom image files unless you delete/move the containing study, and PacsOne will never delete the containing folder under ANY circumstances.
So if you find the folder containing the raw Dicom images are gone, you should check to see if some other user had been using the Window File Explorer and removed those folders/files by accident.