Most common reason is the media has been moved on the hard drive.
You need to relink them.
Thanks - but how do I do this? Do I have to download (add) every single picture again?
Is there a one-step process or do I need to start the entire project over again adding each picture individually?
Download means transferring data from the internet to the computer.
If you double click on the missing file in the Project Assets it will open a window saying Where is file so and so.
Navigate to that file and hit OK.
The rest of the files might follow, might need to do them all separately.
It worked. Thanks Ann!