If you need to convert the video to another format or resolution, you can use software like:
Example FFmpeg Command for Converting to 480p:
ffmpeg -i input.mkv -vf scale=640:480 -c:v libx264 -crf 18 output.mkv
If the MKV file is corrupted or not playing properly, you can try using:
If you're looking for portable versions of software (that can be run without installation), you can often find them on websites like PortableApps.com or directly on the software developer's site.