Brakes suddenly stopped working on Ford Explorer 1997?

If the pedal had been slowly sinking to the floor while you were stopped, that would be a master cylinder going bad. Generally, they'll give you the aforementioned warning before failing completely. It certainly sounds like a master cylinder, and not a vacuum booster issue.

Hey Lisa, When you pressed the brake pedal, did it go all the way to the floor without any tension or was it like normal but just not stopping... Did you or have you heard and grinding or a loud whistling or humming sound or were the brakes pulsing to a stop at any time prior to today? How often did you replace the fluid. and how low was it when you put fluid in it last . Did you notice and fluid spots under your car? and lastly when were the brakes put on last and by whom. Just a list of things you should take to the Mechanic if you should happen to go or call...

It sure sounds like a master cylinder failure. I recommend never adding brake fluid except as part of a brake service. There are only two reasons for the fluid being low - worn brake pads or a leak. Either of those requires servicing the brakes.

sounds like the master cylinder seals have gone so it is no longer pumping or pushing the brake fluid to the brakes.