Can you see more of the dungeon on the map than you can get to? There is a rare bug with dungeon generation that can leave part of a level separated from the rest.
You can use the following to get through walls or into the other part of the level;
Teleporting anywhere (via hex editor)There can sometimes be a small gap where walls meet in the north west corner, that you may be able to toss a teleporter stone through to reach the rest of the level. If you don't have the teleporter stones yet, return to the imp village and explore the little mushroom forest to the north west.
If you have a recent save before entering the act 2 BF for the first time, you could also reload that save and re-enter the BF, which would regenerate the dungeons.
