Use a versatile media player like VLC Media Player which can play almost any video codec and handles corrupted or incomplete MP4 files better than default system players.
If you are trying to open or identify this specific file, you can use the following methods: b677.mp4
This format is universally supported across Windows, macOS, Android, iOS, and most modern web browsers and smart TVs. Troubleshooting & Viewing Tips Use a versatile media player like VLC Media