Ensure the file ends exactly in .mkv . If it ends in .mkv.txt or has no extension, rename it to end in .mkv .
However, without more context about what you're trying to accomplish or what kind of information you're seeking, it's difficult to provide a precise answer.
If the file won't open at all, the "fix" is usually to re-download it, as the data may have been corrupted during the transfer.