Where are the pictures not showing? The import dialog? The grid in the Library?
It depends where your original images are stored. It seems like either you have deleted the actual images from their original location or they must have been renamed and that is why Lightroom is locate those images and ended showing them as missing.
You can try to relocate those images by doing a search for those images by their name.
Here is an article to locate images in Lightroom :
Here is another article that will explain about the workflow of Lightroom :