Internal or external drive for your video files?
When you import media into Premiere Elements, what you get are copies of the originals. But those copies embedded in the project file need to trace back to the originals at the hard drive save location of the originals when they were imported into the project. If you move, delete, or rename those originals after the project is saved/closed, then you are in for some nasty media reconnect issues.
Your first choice is to reconnect the sources with the "Where is the file....?" message "one by one", hoping that you might get lucky and get more reconnects beyond one at a time and one message at a time.
Do you have all of the originals that went into the project?
You could also look at the Replace Footage feature which will reconnect and maintain the edits previously applied in the project.
Please review. Any questions or need clarification, please do not hesitate to ask.