Please confirm what version of Premiere Elements and on what computer operating system it is running?
Am I correct in assuming that you are building this slideshow in the Premiere Elements Editor and not the Elements Organizer Slideshow Editor/Builder?
If you are getting Media Offline messages when you open a saved closed project, the likely explanations include
1. You moved, deleted, renamed files and folders that were imported into the project after you saved closed the project. When you import
your source media into a project, all you get in the project are copies of the originals which are at their hard drive save location. But, the project needs to trace back to the originals at the location where they were when first imported into the project. The "Where is the file....?" messages are your means to make the source media - product reconnects.
2. The other possibility is that you store your source media on an external hard drive and that the drive has malfunctioned, perhaps changing driver letter.
Please review and consider, and then let us know if any of the above applies to your situation.