If the Warriors are caught by a Cave In Event (or something similar), they may find that the way forward is blocked, and they are unable to proceed with the Quest. Whenever this happens, the Warriors can search for Hidden Passages in the dungeon, hoping to find an alternate route.
The rules below are taken from Gav Thorpe's article "Mission Impossible" in White Dwarf #185.
Hidden Passage Rules
A Warrior can choose to search a board section if no Monsters are on the board. The search takes the Warrior's entire turn. This means that usually the Warriors will get 4 chances to find a Hidden Passage on each board section, pretty good odds! Once a Warrior searches a particular board section, they cannot search it again.
When searching, a Warrior rolls below on the Hidden Passage Table. Elves have a knack at finding Hidden Passages, and add +1 to the roll (although a 1 is still counted as a 1). After searching, reposition the Warriors using the Search Rules.
Hidden Passage Table
1. Collapse! Each Warrior on the board section that was being searched takes 1D6 Wounds (unmodified) due to falling rock and rubble.
2, 3, 4. Solid Rock. The Warrior cannot find any Hidden Passages in this board section.
5, 6. Found it! Attach a Doorway to the board section and place the remaining unexplored Dungeon Deck behind it. Each dungeon will only have one Hidden Passage, so there's no need to continue searching after this point.
No Way Out?
If the Warriors fail to locate a Hidden Passage, they must make their way back to a Settlement since there is no way to progress on this Quest. They can then seek out a new adventure. If you want to keep the current adventure going (if it's part of a campaign, for example), the Warriors consult with their patron. The patron informs them about an alternate entrance that leads to the Objective Room. Begin the adventure afresh with a new Dungeon Deck, but add an additional 1D3 Dungeon Cards to the top of the deck!