magatama problems in shin megami tensei notcturne

I don't know if anyone here has even heard of the game shin megami tensei nocturne but if you have I would like some help with something. I am pretty far in the game and I have enough strength to open the cell door down in the mantra prison. The magatama gaea is located in the cell. I didn't go down to the cells untle after the scene with chiaki where she turns into a demon and when I did the cells were gone! the treasure chest was still there but it was empty. now I cant get the gaea magatama from the cells but I was hoping that maybe its just been moved somewhere and i can go get it somewhere else. If anyone has some info on this i would be grateful