Advanced tools like Untrunc or Clever Online Video Repair require a "good" sample file—a working video recorded with the same settings—to reconstruct the missing data in the "bad" file.
Always check the size. A high-definition MP4 of a 3-hour movie should be at least 2GB. If you see a file that is only 100MB, it’s likely a low-quality compressed version or a sample clip. Common File Formats & Their Challenges Titanic Index Of Last Modified Mp4 Wma Aac Avi Fix