please help me. i'm going mad.

i've finished the quest line for the dawnguard dlc.
now i don't have the initiate's ewer anymore and can't gain acces to the hidden room in the inner sanctum.

what happens to the initiate's ewer once you' are done and is there a way to retrieve it? what should i do?

It probabely locks off after going there for the quest.
Not got the dlc, but it sounds likely.
Or it's a bug.

It should still be where you stored it or in the inventory of the merchant you sold it to.
It does not just despawn.

What did you do with the ewer? Nothing happens to it, it remains in your invenrtory after the quest finishes, so if it is not there you did something with it.

As long as you have the ewer, you can go back to that "secret" section. I know you don't have the dlc so you were just making a estimated guess but I just wanted to clear that up in case the OP got the wrong impression.

No it becomes a non quest item allowing you sell it or lose it.