We have no idea because we don't have any idea what you did. It's either user error or you are not using the most recent version of the software. Nobody can give you and answer to that question without a lot of details of your workflow, your experience level. If this is the first time you have ever used AE then the first thing wrong with your workflow is that you didn't take the time to learn the basics of how to use the software. Basic AE
If you are an experienced user and have successfully produced a shot in AE then I apologize for not understanding your workflow and coming up with an answer more specific than make sure you know what you are doing and make sure your software is up to date.
I would first have to point out that I have been successfully exporting video both from the render file and through the Adobe media encoder, literally hundreds of files, and then suddenly all I get is a black screen as the final export product, though I use the same process that I have always used.
This now occurs both in exporting from the render file and from the media encoder. I'm wondering if I may have accidentally hit some function in the layeror panel or the composition panel that blocks my composition from being visible for export.
I have some skill with Adobe aftereffects, but I am by no means an expert and certainly have it mastered the technical vocabulary.