Its hard to guess without being able to do some basic diagnostic tests. Did it lose power and cut out and fail to start or does it run and just have reduced power? If it failed on the freeway the timing belt could have broken, the fuel or ignition system have lost a key input needed to keep the engine running. The fuel pump could have failed or many other things. If the engine "revs" strongly even the transmission could be at fault but there are too many possibilities to just guess.
Car started sputtering then stalled. Tried to restart but would not. Waited About 20 minutes and car started again. Similar thing happened few months earlier where car sputtered but didn't stall. This is a Hyundai Sonata 2005 V6 and this happened in July 2013 then again December 2013. Ended up being Crankshaft Position Sensor. The thing was worn and wires were exposed. Part is inexpensive and had friend replace it. Problem solved.