I have a 2004 mustang . Car won't start. New battery,starter turns,and I have about 2 gallons gas.

Okay, yes Ford's, the '04 Mustang included do have an inertia switch to cut fuel supply to reduce the risk of fire in the event of a crash, anything enough to deploy air bags will set it off. It always has a red re-set button and they are in different places,,,the Mustang often in the trunk. The shut off valve and re-set are not necessarily in the same place.

Click on these images and study them

2 gallons of gas, if there is really that much in the tank will start the car

.