What version of Premiere Elements are you using and on what computer operating system is it running - 11, 12, or 13.
The disappearance of the Project Assets Panel has been report a few times. I do not know that there is one solution.
The following is some troubleshooting that I have suggested in the past
Please make sure that the only program that you have open is Premiere Elements
First nudge to trigger Project Assets to open in Expert workspace....
Go to the Quick workspace. Add Media/Files and Folders. Do you get your selected file in a scene space in the Filmstrip of Quick workspace? If you do, switch to the Expert workspace and select Project Assets. Does it open displaying the file that you imported into the program in the Quick view?
Second nudge, if necessary, to trigger Project Assets to open in Expert workspace...
With your file say in Videos on hard drive, cascade the window with Videos and Premiere Elements Expert workspace window...drag the file from the Video's Folder into the Expert workspace Timeline. Then determine if Project Assets will open to display a thumbnail of the file that you just dragged in for import purposes.
Please let us know if that worked for you.