Sometimes an entrance can be partly or completely obscured by a wall. Check the auto map, to see if you can see some blue sticking out somewhere. Running around methodically searching with the mouse can also help.
Occasionally the entrance can be a pile of rocks, rather than the usual fountain thingy.
Rarely part of a level can be separated from the rest (which sounds like the case here), which could contain the entrance to the next level. There are a couple workarounds, though.
If you gather up some crates, packages or barrels, you can use the procedure in the topic armory trick in act 1
to get a summoning doll through a wall, have it drop a teleporter stone (only available in acts 2 and 3), and then teleport your characters through. You can get the teleporter stones in the mushroom forest north west of the imp village.
There can sometimes be a slight gap in the north west corner when two wall meet, which it may be possible to simply toss a teleporter stone through. I'm not sure if you can get a teleporter stone through wall cracks (in DD you could get teleporter stones through metal gates and open windows), but you could try playing around with where your character stands and where you try to drop a teleporter stone.
Alternately: Teleporting anywhere (via hex editor)
Also, you could just skip the rest of that dungeon. You may miss one or more BF quest solutions, but completing the BF dungeons is not necessary.