LPG Car It will start on fuel when turn to lpg it runs for three seconds the stops suppling gas then cuts out?

The car was running fine till i had a blow back happened, This blowback happens in intake part of fuel delivery. once this happened this gas sytstem never worked again since. But is will come on for 2-3 seconds then you here a relay switch of then the car stops. what can be the system fault be, that automatically switches the fuel (lpg) supply off? If any other information you think is relevant to this problem, let me know.