* Is the fuel gauge not working correctly? This is a sensor or gauge issue, not something solved by a button.
* Is the car not starting because it thinks it's out of gas? This might indicate a problem with the fuel pump, sensor, or other fuel system components.
* Are you trying to reset a fuel economy average? Many cars have a trip computer that can reset average fuel economy, but this is done through a sequence of button presses on the instrument cluster, not a single "reset" button.
Provide more details about the problem, and I can give you more specific advice.