I have that happen a lot. It's frustrating to me to as a DM since I have to wing-it. I can wing NPCs well, but I'm not very skilled at coming up with an alternative scernario, My players in particular are pretty wussy, especially for vet. players, "What a vampire? We're first level, forget that. Let's go kill some goblins." They fart around for an hour or so, ad nadiseum (*example from Crucible of Freya, where it's only the rumors of a vampire). Of course, I've learned to lure my players with gold, that gets their adventuring spirits up.
My trick is to involve them realistically; no person is an island. If players don't want to go into the adventure, that's fine. Maybe a rival group of adventurer's finished the quest and now they are carting off a ransom in gold; jealousy can be a powerful motive. Or, better yet, if it's not too late in the going, you can include rumors of rival groups; the actual quest not completed. Or, as others mentioned, maybe the band of orcs grew larger. The orcs then destroyed the town, chopped the forest, and stole the gold mine. The players may have to rid themselves of an orc infestation (and build a new barracks and town hall).