My once black cast iron skillet is now dark silver on the inside. Have I ruined it?
It sounds like you may have scrubbed the seasoning off your pan.  It is not ruined, but will need to be re-seasoned.  As you can see on the right (if accessing the site on a computer), where it says "related", there are a number of links to questions and answers about cast iron seasoning.  If you are accessing the site on another device, you can search for "cast iron care", or "cast iron re-seasoning" using the search bar.