"I downloaded an a movie and tried to play it in Windows Media Player. I can hear sound, but the picture is just black. Is there something wrong with the movie?"
Hearing sound from a video but not getting a picture is a fairly common occurrence. Playing a video file in Windows Media Player (or any other media player software) requires two software components called codecs. One codec handles the video portion of a movie file, the other codec handles the audio portion of the file. When your computer is only playing the audio, that means you have the required audio codec, but your computer lacks the necessary codec to make the file play properly. To solve this, you need to first identify what codec is required, then download and install the missing codec.
Identifying the missing codec
Before you can get your movie file to play correctly, you need to identify what's missing from your computer. My favorite solution for finding missing codecs is MediaInfo, which is a free download. You need to download MediaInfo and install it to examine your movie file.
In addition to telling you the specific codecs required for the audio and video portions of a movie file, MediaInfo also assists in verifying other potential problems with video playback by comparing the expected length of a file against the actual file size and preview the movie file to simulate playing the movie to determine if the software can render the file correctly.
If MediaInfo determines you are missing a codec, it will generally point you to a location where you can download the solution. Once you download and install the codec, you should be able to play your movie with no further issues.