The advanced version product may not run in previous version as the technology is different.
Please check the technical specifications too.
Please give more information about what is involved....
a. Do you have a Premiere Elements 7 project file (.prel) that you are trying to open in Premiere Elements 14?
b. Do you have an export from Premiere Elements 7 (movie format) that you are trying to import into a Premiere Elements 14 project using Premiere Elements' Add Media/Files and Folders?
There is no guarantee that an project file created in an earlier version will open in a later version. And, if you try to do this, please work from a copy of the Premiere Elements 7 project file as a safety next.
At this time is Premiere Elements 7 and 14 on the same computer and are the Premiere Elements 7 source media still where they were when they were imported for the first time into Premiere Elements 7? Did you delete or change names for files and folder of the Premiere Elements 7 project after the project was saved closed?
Let us start here and then decide what next based on the details in your reply.
Any questions or need clarification, please do not hesitate to ask.