Premiere Elements 14 on what computer operating system?
When you import files into a project, all you get in the project are copies of the originals which remain at their hard drive save location. But the copies in the project need to trace back to the originals where they were when first imported into the project. If you move, delete, rename files and folders that went into the project after the project is saved closed you get media offline situation with those "Where is the file ____?" messages when you try to re-open the saved closed project.
Another possibility is that you stored the originals on an external hard drive and the external hard drive is malfunctioning or changing its drive letter related to the external hard drive being unplugged and plugged in again.
Please consider, supply more information, and then we will decide what next based on the details in your reply. Those "Where is the file____?" messages are you remedy to correct most of the situations described.