Cast Iron Skillet with Lid Rusted togethe
There is a product called Naval Jelly that is designed to remove rust from metal items.  Its available in our area at most hardware stores.  Menards has it. Use a toothbrush to work the jelly in between the lid and the skillet and let it sit for 10-15 minutes.  It literally changes the rust into a water-soluable paste that you can wipe away with a clean, wet cloth.  Then you will be able to season your skillet (and lid) so it can be usable again. Hope this works for you.