Problem: You thought that you had downloaded the video. However the video did not download all the way. This can happen if there is a disruption in your internet connection during the download process.
Solution: Download the video again. Here is how you can check that: Open the downloaded video file in VLC media player. Then grab the slider bar with your mouse and move it almost all the way to the right, to the end of the video. If no image shows up in the video, i.e. you just get a black screen, then the video has not downloaded all the way.