The sewer golem shouldn't disappear even after its function is complete, it just stands around afterwards. If you've waited awhile and nothing's happened and there's no sign of it in any of the corridors (possibly trapped behind some goblins) I'd consider that a bug and try restarting if I were you.
Yeah check all the corridors leading to the central room, it may have got stuck on the Goblins or a quasit as Pantalion stated. That would be my guess, if not then you could try and console in the Orb thingy and another Sewer Golem, assuming you have the console enabled.
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