Hundreds of things. Crankshaft position sensor, camshaft position sensor, ignition coil pack, Automatic Shutdown (ASD) relay, corroded wiring connector pins, engine computer, body computer, any of the dozens of computers shorting out the data buss, plugged fuel pickup screen, overheated ignition switch contacts, melted connector terminals for the ignition switch connector, running out of gas, battery was recently disconnected or run dead, fuel pump relay, dead or intermittent charging system.
Most of these things have other, related symptoms that provide valuable clues. How quickly the engine stalls is a clue. What have you checked so far? Any other clues or observations? Does the Check Engine light turn on while driving? Will it start right up again or do you have to wait for something to cool down? You have to provide some information and be willing to perform some tests for us to help figure out the problem.