This has happened because you have moved the folder that your images were in (or the images have been moved). The software does not embed the images in to your project file. It pulls the images from where you have saved them.

You will either need to restore the images back to their original location or you can relink each of the images. It is also advised that you do not work from an external hard drive but rather copy the images to your machine so that they are available locally.