When you import files into the Premiere Elements 13/13.1 or Elements Organizer 13/13.1, all you get are copies of the originals that remain in their hard drive save location. If you get Media Offline notifications, that can mean at least 2 things....
1. You moved, deleted, or renamed folders/files that were used on the Timeline after the project was saved closed.
2. You store your source media on an external hard drive and the external hard drive has malfunctioned overall or something has changed it drive letter.
The critical question is can you still find those originals in their hard drive save location? Can you find them anywhere on the computer, including the Recycle Bin?
If you still have the content, you can use the "Where is the file ______?" messages that you get when you reopen the project and do the media - project reconnects. It can be a complicated or not so complicated.
Please review the above and let us know what of the above applies to your situation.
Any questions or need clarification, please do not hesitate to ask.