In order to save data storage space on your device, Film Story does not keep copies of the movies when movies from the camera roll are used on the app. For this reason, when the original movies are deleted from the camera roll, they cannot be found on the app; as a result, such movies cannot be played and “File Not Found” message pops up.
Moreover, as for pictures, when importing pictures into the app, they are converted to movies and if this conversion process fails, you will get File Not Found as well. It is supposed to be restored automatically, however, if the issue is not resolved, please delete the picture and try importing it again.