Hard to tell from your description. Is the battery good? When the battery gets below a certain voltage, the car's computer can't function, and the car will just shut down. Next, if the battery is good, did the car overheat? Odd for a car to lock-up, but Ford 3.0L motors have been known to do this without warning on occasion, although it is not typical. One quart low won't lock up a Taurus motor, but low oil pressure or severe overheating will. Finally, is the starter frozen? (It's a long shot, but it's possible). Pull the plugs, and put a socket/breaker bar on the crank pulley and try to turn the motor over, before and after disengaging the starter. If it won't budge, it's locked-up. BTW, check oil and coolant for contamination. The engine could be hydro-locked if you have a bad enough head gasket leak and the cylinder(s) are filled w/coolant. Probably not, but I'm trying to cover all the possibilities.
If you check battery voltage and it is fine (over 11.5 volts), and and the car is definitely locked-up you have the time and want to know the answer to "what happened", there is a definitive test you can buy at Carquest or Napa stores for about $13.00. Drain some oil from the pan, and capture at least a cupful of oil (but not the first or last quart). Buy the Oil Analysis kit from the parts store, fill the 6 oz. container, fill out the form, and mail it off. In 1 or 2 weeks, you'll get a full analysis of what EXACTLY is going on in your engine, with a detailed report of IF and WHAT went wrong. Cool, huh? It will also alert you to pending issues that may be of concern, if your engine turns out not to be terminal.
