Security system reset

If the security system is doing that, the engine will stop after two seconds. To run for more than that, you have to treat this as a normal stalled engine problem. Since you didn't list the engine size, I can only suggest to listen for the hum of the fuel pump for one second when you turn on the ignition switch. If you don't hear that after the stalling has occurred, the pump is a good suspect. Your mechanic would start by reading the diagnostic fault codes. If you have a code reader, you can do that too. Also check for loss of spark. Most crank / no start problems involve a loss of spark AND a loss of injector pulses. Typically that is caused by a failing crankshaft position sensor or camshaft position sensor.