I've had similar things happen before; a friend's group refers to it as "volcanoing" the adventure, because the first time it happened they used a wish spell to cause the villain's inactive volcano lair to erupt. You just have to adapt (sometimes taking a 15 minute break in game) and figure out what the consequences would be. Obviously the PCs aren't going to get any XP or treasure. The "secret" within the house will remain undiscovered, and might actually benefit those involved. It will most likely upset the villagers, because unless the party convinces them otherwise, the ghosts will be free from the site possibly coming to the village!