So, I finally defeat Mercer Frey, abuse the hell out of the newly gained skeleton key and now I'd like to return it... theres a slight issue though.
I CAN return t but there is NOTHING I can do after that.

Once I returned the key the thing called forth Nocturnal and she gave me a speech but after she goes back into her realm, it is frozen. The mission no longer progresses and I'm stuck in the small room and cannot do anything. Hell, I cannot even draw my weapon. The mission still says to "Listen to Nocturnal" which I gust did so... now I am stuck, theres nothing or no one there! And since the mission didn't progress, I cannot do anything.

Is there some way to fix this?
Or a way to get around it?

The only way could be reloading a previous save, since the game sometimes just glitches randomly and reloading a previous save normally fixes it.

I hope you're not relying on autosaves and saving manually and often.

I had a few problems with this one too. Karliah was invisible the whole time inside this small room.
I know you're suppose to choose one of Nocturnals powers. They should show as moon cymbols on the floor. There are 3 of them. After you choose one, Karliah talks for a bit and then you just walk through one of the portals and it will bring you to the entrance of Nightingale Hall. If the cymbols don't show after you put the key into the center keyhole on the floor then you will have to re-load and try again.

I manually saved just before entering the Inner Sanctum area. I didn't even see Karliah in there, I tried standing on the symbols and walking through the portals. It seems it just doesn't want to register that I did "Listen to Nocturnal" so, I reloaded the game a few more times.... I think it is just stuck here. >__< I could really use the shadow power too...

did you go get a special ability from one of the moon's on the floor? after, Karliah should appear, talk to her, then you should be able to leave.

Try reloading a previous save that seems like the only solution if the game is glitching on you at this point.

Even with reloading from an earlier save, it seems that it didn't work so I guess that I will just be keeping the Skeleton Key until there is a fix for this.

A fix won't work if you already save the game with it like this. The patches only prevent the issues. They don't fix them.

I managed to finish this but like I said, Karliah was invisible the whole time in this room. I only knew she was there because she was talking to me.

If I go back there now, the cymbols don't even show up on the floor (even though they still work) It went back to the way it looked when you 1st go there with the keyhole in the middle of the floor. Karliah is also suppose to live at Nightingale Hall. I've been there a few times since and she is never there. The only time I've seen her since is during the Guild Leader ceremony. Other than that she is like gone from the game.

Maybe Louis Letrush can take her place! There are plenty of him to go around!! lol

this happened to me, what i did was wait for a couple of hours if that doesn't work wait another 24 hours and it should get moving again. try talking to karliah before she goes out the door.

I've been holding on to that sweet skeleton key, too, so thanks for the warning!

Same thing happened to me. I waited for a few hours and then the quest pushed forward.

One more of this game "qualities" is that relies heavily on the cache system, but do not releases data so if after the second try is still glitched it would be advisable to do a restore system and rebuild database from the PS3 recovery menu to fully clear the cache.

I hope it helps.