I think that's the room where they ask you to find some way to open the gate, right? There should be a side passage which you need to follow for a little while. Was there also something on a pedestal that opened a secret room with some treasure and a trap? You don't need to go down there to solve this puzzle. Look for a lever on the wall, I think it was on a pillar overlooking the lower room.

When facing the gate they want you to open, turn to your right. There is a side tunnel. Follow that tunnel up some stairs and follow the hallway. You will see a pedestal to your right (which leads to the secret room you described, I believe). Walk past it to the end of the hall and turn a corner, and you will see a bridge which crosses over the lower room. At the end of the bridge you will find a rectangular room with a couple of shelves and a chest at the end. Ignore them for now, and look to your left. There will be some candles near a pull switch on the wall. Use that, and you'll be ready to continue.