The inertia switch is a safety device designed to cut power to the fuel pump in a collision. This is usually located under the dashboard or in the trunk area (check your owner's manual for the exact location in your vehicle). Resetting it (pushing the button) will usually restore fuel pump power.
The fuel pump relay is an electrical component that controls the power to the fuel pump. If a problem exists within the fuel system, this relay could also interrupt fuel supply. Its location is typically in the underhood fuse/relay center (again, consult your owner's manual for precise placement).
In short, there's no simple on/off valve like you might find on some older vehicles. Any fuel shutoff will involve working with safety features or electrical components.
