If you are not able to unzip/unpack your files, but you are able to see a list of files when you look into the file, then there is a small chance that you will be able to recover your files. If you get the error message, "Can not open file 'a.7z' as archive" then there is also a chance you may be able to recover some of your files. You will need to try the Z-Zip recovery procedure.