If your standard video player (like Windows Media Player or QuickTime) won't open it, use a universal player that supports MKV:
If you need the file to be a standard MP4 to use on a phone or TV, you can "remux" it (change the container without losing quality):
This usually happens when a file is renamed or converted improperly. The computer will treat it as an because that is the final extension. MKV is a "container" format that can hold high-quality video, multiple audio tracks, and subtitles. 2. How to Play the File