Nothing, really. I normally don't have 'the' quest hook, but 'a' plot hook, or rather several plot hooks.When the PCs refuse to pick up the quest hook, what do you do?
I don't plan very far in advance except for a very general outline of events. If the players show interest in one of the plot hooks I've planted, I'll start to develop it in more detail.
If I have some 'really cool (TM)' ideas for some hooks, I'm not below recycling them, i.e. they'll stumble over slightly re-dressed variants of the hook until they bite.
But I'm rarely that invested in a particular idea, unless it's very late in the campaign and I want to draw things to a satisfying close.
From time to time it's happened, that my players decide to abandon an adventure at some point - usually because they get the (sometimes correct) impression it would be too tough for them to continue. This is a case where I don't typically let them get away without some negative repercussions later in the campaign. Often it's possible to re-use at least some of the planned encounters in a different context to avoid having wasted the time to prepare them.