Your student just submitted a long video. You went to the grading tab right away, and you can only play the audio. You see a black screen where the video should be.
What's going on? Probably nothing: videos might take some time process, depending on how much traffic there is on the system and the device used to record (the newer the phone, the higher the resolution).
If this happens, wait a few minutes, refresh, and you will see the video appear!