Right-click the file and use your antivirus software to scan it before extracting.
The download likely failed or was interrupted. Try downloading the file again.
(e.g., software, game mod, puzzle, documents)
Ensure you copy the password exactly as provided by the original source, as it is case-sensitive.
If the file is in the newer RAR5 format, ensure your software (like 7-Zip or WinRAR) is updated to the latest version. To give you more specific help, could you tell me:
Files from unknown sources can sometimes contain malicious software.