For starters, don't let it slam down without a cable attached!!
Is it in the water, trailer, stands? It's hard to answer without knowing that. If you are able, I would try to replace the cable before doing anything else.
Just did the keel removal. Mine was sticking because it had rusted and swelled up to where it was hanging on both sides of the pocket. Mine would come down but sounded like grinding gears in an old Ford pickup. Make sure you have a cable attached, then reach up from the back side and pull down ont he cable. Once it breaks free it should move all of the way down. Be ready to replace the keel pin and brackets. The cable is 10 feet of 3/16 stainless or galvinized, with an eye and a 5/16 shackle to attach it to the keel.
Dealt with the stuckup keel as well. Had the boat on stands when we freed it. It was a combination of rust swelling and not so fresh seafood. We used a long flat thin piece of metal to remove the sea life and get it free.