Started sputtering but still drove, then a few days later was driving and it died and would not restart. Left me stranded and I am still stranded. Need suggestion what it could be

The car sometimes didn't start at all. It stopped at the middle of the freeway driving on 60 miles/hour. Dropped the car at Hyundai dealership, they hold the car for 2 weeks.

First I thought it was full tune up I need. I did but the problem was worse. Then I did flush the transmission and the problem is still there.