Damaged files
Distinguish corruption from encryption.
An opening error does not always mean encryption. Keep the original, note the format and error, and look for a working copy or earlier version.
Check whether the download or transfer completed. For a multipart archive, verify that all parts are present. A password prompt may simply indicate normal password protection.
Run checks on a copy and never overwrite the original. If a storage device disconnects, makes unusual noises or reports read errors, stop repeated attempts and seek specialist help. Open recovered files in the appropriate application and inspect their contents; a tool reporting success does not prove that all data survived.