thieves guild problem (possible Spoiler)

[MatiasDet]

I'm at the quest in which you have to enter a castle, then enter the room of the court wizard, find a secret passage and go trough it until you get to a cave from which you're supossed to retrieve somethig that belong to the wizard. Well, i was doing all right until found miself with a locked door that couldn't be lockpicked, and could only be opened by suing a key. This brought to me the thought of having to "take" the key from the wizard, BUT, he is nowere to be found. Does anyone remember this mission well enough to tell me where he is or if i have to do something else like "avoiding" the door of sth like that? Thanks from now.

[qizatch]

While I have not done this particular mission, most of the time when you're not able to progress in a quest, you've failed to talk to someone. So start by checking your quest info and make sure it really tells you what you think it does.