One theory is that they drowned in the Great Flood, the one avoided by Noah and seven of his family members, along with a boatload (get it?) of birds and animals hand-picked in seven pairs of twos to repopulate the world.
Maybe the dinosaurs were too big to be included. Maybe there were just too many of them.
For an expansion on a literalist/creationist take on things, go here. I was trained to answer people who talked about carbon dating with a “Maybe God made the rocks look older than they actually are.”
And thanks, Tim, for the link.