Well, i received the skeleton key and did a fuc*"* mistake..... i opened the doors but retreated to continue to explore the dungeon ( Misery ). When i went back, i found out that the doors are closed and that i need to forge another key. I forged another key with the Blood of CH'THON & other ingredients but still the door won't open. Any help guys????

The ghost fella that give you the first skeleton key says the doors only open once. After that you need to come back another time. To translate that into non-lore terms: it means you need to start a new game session to open it again.