Sounds like you moved the source media that went into the project (project.prel) after you saved/closed the project. That is the most common cause of those nasty situations with Media Offline colorful thumbnails and Missing Media Messages. There are others less common, like the drive letter changing on an external drive being used to store the source media. Lot of possibities to explore and see what scenario fits your situation as Hunt suggests.
If you moved the media after the project save close, we can review with you how to use the Missing Media Messages to recover or for that matter the Replace Footage feature for that purpose.
For future reference you might want to look into the advantage that Project Archiving can offer to avoid the situation that is probably causing your issue.
Always good to give the Premiere Elements version and computer operating system being used so that the suggestions can be made version specific if necessary.