source files?

Hi, I'm new!
First experience with Movie Maker 2; got photos w/music and go to copy on cd
and this message appears:
"This project contains source files that are missing or that have been
modified after import. Your project cannot be saved as a movie until the
source files have been located or deleted from rhe project."
What have I done and/or what can I do to save my days hard work????

Please help and give me step by step...I'm just a beginner on this.
thanks!