Sometimes files are corrupted during transfer. Try the FixVideo Online Tool or Restore.Media if the file was from a camera that cut out.
In tools like Adobe Premiere Pro or DaVinci Resolve , ensure your "Color Space" is set to Rec. 601 for SD timelines. 601 mp4
If you are using downloader scripts like yt-dlp , ensure you are on the latest version, as bugs often cause "unable to download" errors for long videos (common in 600+ MB files). Sometimes files are corrupted during transfer