Cadillac WILL START BUT NOT RUN

I DROPPED OFF MY 2004 CADILLAC CTS, 3.6 LITER, OFF AT MY LOCAL WAL MART STORE FOR AN OIL CHANGE. IT WAS RUNNING FINE WHEN I LEFT IT BUT AFTER THE OIL CHANGE THE CAR WILL START AND RUN FOR APPROX. 3 TO 4 SECONDS AND THEN GO DEAD, LIKE IT IS NOT GETTING ANY FUEL. YOU CAN LET IT SET FOR A MIN. OR SO AND IT WILL START RUN FOR 3 TO 4 SECONDS AND GO DEAD AGAIN. WHAT IS THE PROBLEM WITH THIS CAR. I HAVE CHANGED THE CRANK SHAFT POSITION SENSOR.