Premiere Elements 10 on what computer operating system? What video card/graphics card does your computer use?
Please explain when you say that your content is gone...
Have you saved, closed, and reopened the project to be greeted by "Where is the file _____?" messages?
Have you saved, closed, and reopened the project and found a Timeline with black images instead of your imported files (no "Where is the file ______?" messages)?
Are you storing your source files on an external hard drive? Drive letter changing?
After you save close the project, are you moving, deleting, or renaming files and folders that were imported into the project?
Do you have the latest version of QuickTime installed on your computer (7.7.8 if Windows 7, 8, 8.1, or 10 64 bit and 7.7.6 if Windows 10 64 bit).
Please consider and supply details. Any questions or need clarification on anything written, please do not hesitate to ask.