In order for us to customize troubleshooting for you, please tell us the version of Premiere Elements that you are using and the computer operating system in which it is running?
Are you moving, deleting, or renaming files folders that were imported into the project after you saved closed the project?
Are your files saved to an external hard drive or to a location on the Local Disk C?
Do you have more than one version of Premiere Elements on the same computer? If so, are you creating a project in an earlier version, editing it in a later version, and then trying to edit the later version edited one in the earlier version again?
The Adobe document on damaged projects cited in post 1 may be of help. So, please review that also.