What can cause a loud sound of breaking on a front end of a car?

I have a 2002 Mercury Sable, in pretty good shape. I was getting in the car today, got in and heard a horrible gl like breaking sound from under the car. I got out and looked, did not see anything in particular. I then got back in started it up hit reverse and the front drivers side tire went flat. i tried to look around to try and see something under it, but couldnt see anything. I dont know if the front end has an axle or something that can break that would do that? this just happened so its still parked at my work parkin and haven't changed the tire yet. tomorrow we are going to look under it and try to see what happened, i was just looking for some type of hint tonite