Which version of the program are you using and on what operating system? Were the projects created by the same version of the program?
How are you opening your project? You open a project by going to the File menu and selecting Open.
This should not be confused with importing media into a project, which you do by clicking Add Media. You can't open a project using Add Media.
I have already answered your thread question this morning when you posted the question in the thread of another.
Please refer to your post today in that 2014 thread (2015 - your post 5 and my post 6).
For your convenience, I wrote
What computer operating system is involved?
What were the export settings that you used in Premiere Elements 12 to create this movie file that cannot be imported into Premiere Elements 12?
Just in case request....please verify that you are trying to open an export file from a Premiere Elements 12 project file and that you are not trying to open a Premiere Elements 12 project file in another Premiere Elements 12 project file.