Premiere Elements is not backwards compatible. You can not open projects produced in older versions of the program with newer versions of the program. It's possible in doing so you've corrupted your project and now it won't open in either version.
But always complete your project in the version you started it in. Or, if you want to try to open it with a later version, make a back-up copy.
If you've lost a lot of work, we can suggest ways you may still be able to recover and finish your project in version 2018.