1998 Mercury Villager front disk brake seizing up

I have a 98 mercury villager van, and the front disk left side brake, lock up or should I say tightened up on me today when I was doing some heavy braking, delivering mail. I could place the van in neutral and it would come to a stop and I could hear the front brake squeal. It it a caliper thing or a master cylinder thing? AND what would you do to fix it? Thanks! JB