I've updated level map in
this thread
To help you there are the things I believe your PCs may guess they could still do in the level:
1. Go through west doors
2. Crawl to the monster standing at the Iron lever to check what the lever does and if the room where monster is has any other doors/exits (i.e north)
3. Reach the strange stairs up at the end of this long room - they are probably leading somewhere
4. Press the lever here and see what it does
5. Try to open the thing that is blocking you from crawling through this (will require two hands and decent strength to lift the sheet)
Hint: there are more places in this dungeon you failed to discover for variety of reasons. I can spoil you some of those reasons if you want spoilers.