A flat battery is one possible reason a car won't start, but it is not the only one. Check for warning lights, loose or corroded battery terminals, and whether the engine cranks when you try to start it.
If you use jump leads or a jump starter, follow the vehicle manufacturer's instructions. Incorrect connections can damage vehicle electronics, and some vehicles require specific procedures.
If the battery is damaged, leaking, swollen, or frozen, don't attempt a jump-start. Arrange professional assistance instead.
If the car starts after a jump, have the battery and charging system checked so you can identify the underlying problem.