The most common causes of black video (only audio) are:
- Not having the correct video codec to decode the video file. This can happen with certain file types. While it might be possible to remedy this by figuring out what exact codec the file needs and then getting that installed on your system, this can be fraught with peril. My recommendation in these cases is to convert the source video file to a supported file type (preferably .wmv). I've used Freemake Video Converter for this task, but feel free to research your options.
- Having an outdated video graphics driver. The solution for this is to run dxdiag and go to the Display tab. Find out the make/model of your graphics card along with the installed driver version. Then go to the manufacturer's website and find and install an update. Alternatively, you may have to go to your PC manufacturer's website to get a graphics driver update, if they prevent you from installing driver updates directly.