Locked out of The Black Cathedral?

I finally found Kulemaks invitation to the Black Cathedral. I fought all the way to his final form but I was having trouble and it was getting late so I logged off.
The Black Cathedral entrance still shows on the minimap in the Well of Souls but I am unable to jump in again, nor do I have the invitation still.

Is this a bug, or do I need to find another invitation again?
If I do, will this invitation even drop again if you've used it once before?