Open the folder containing bigfile.000 . Look for:
Here is a comprehensive guide to diagnosing and fixing this error so you can get back to your game. 1. The "Clean Slate" Fix: Delete and Revalidate Unable To Open Bigfile Bigfile.000
If you know the original file type (e.g., it was a ZIP file), you can try to rebuild the header using a hex editor like (free, Windows) or Hex Fiend (macOS). Open the folder containing bigfile