Make sure that your project settings match your video.
Please revise your thread title to reflect the question that you asking. And, also please give details for your issue which are presenting as a mismatch between source media, project and export settings.
1. What version of Premiere Elements are you using? What computer operating system?
2. What is set for the project preset of this project that produced the video with black borders?
See Edit Menu/Project Settings/General and the readings for Editing Mode/Timebase/Frame Size/Pixel Aspect Ratio (even if the fields look grayed out).
3. What did you set for the export settings in the sharing area of the program? When you got to the preset for the export choice, did you use the default settings or did you customize the preset under the preset's Advanced Button/Video Tab, Audio Tab and, if applicable, Multiplexer Tab.
What do you want to do - know how not to do that again or how to remove the black borders from an already exported video?
Please consider, and give details.