.zip files greyed out

Bring up the Info pane for one of the zipped files and make sure the "Open with" is set to the Archive Utility application.

If it's not then select other, navigate to Macintosh HD/System/Library/CoreServices/Applications folder and select the Archive Utility.

Then click on the Change All button.

Log into another user account on your Mac and see if you can open the zip files there.

If not, then boot into Safe Mode, Mac OS X: Starting up in Safe Mode, from your own account and try from there.

Select one of the grayed out zip files, press "Command + i" and see if it's locked.

So-called “zip” files now come in multiple flavors: .zip, .gz, .xz, .tgz, etc, while compressed files have a .Z extension, and RAR files have .rar. What extension actually appears on that grayed out “zip” file, or has the Finder hidden (Get Info) it?

A damaged “zip” file will either be entirely ignored by Apple's Achive Utility, or it would throw a dialog that the file is damaged, or not in a readable format.

have you checked if you need to update the zip apps the unarchiver or Stuffit expander